Now is the time of year when bookstores have their most-complete selection of new guides, especially the standard series from U.S. publishers that reappear annually. The choice is wide, and it's not always easy to decide which is the best guide for you.

The standards, found in almost any large bookstore, include: Fielding's, Fodor's, Frommer's, Fisher, Birnbaum's and Harvard Student Agencies' "Let's Go."

The series share many characteristics -- all emphasize Europe -- yet they have many differences, too. Two are directed at budget travelers; two others are looking for travelers with fatter wallets; one makes a special appeal to sports- and recreation-minded travelers, and those with special interests; and another tries to provide a bit of everything.

Among them are series to turn to for help in finding places to stay and places to eat; the value of others is in the historical and cultural insights they offer to another city or country.

There's no such thing as a perfect guide. It's wise to consult several to properly prepare yourself for a trip, especially if it's to a foreign destination and if it's your first time there.

There are a number of all-Europe guides on the market. These are handy if you are trying to decide on a country or are planning a 15-nation sweep in two weeks. You don't need much specific information, and you don't get it from these massive volumes.

If you know where you are going, you are better off consulting guides to individual countries, or regions within a country, or to individual cities. The inquiring traveler will go on to find even more specific books on history, art, music, politics.

But you must begin at the beginning, and these six readily available series offer a good start. Each is described below in what can be called a guide to the guides. No attempt has been made to rate them. Volumes within a series can vary in quality depending on the author or authors.

The intent is to help you reach for the right books for upcoming trips.

* LET'S GO: The backpacker's guides to travel on the cheap, the very cheap. They are written and edited by Harvard University students primarily for students on a tight budget.

And how could you doubt it? In the volume on Italy, there is an entry for Venice that points out that the train station floor is a last-resort possibility for a night's sleep when the city is full.

"It's reputedly safe," advises the 1985 edition, "and the cops won't bother you until morning, when they'll kindly wake you up."

An extreme example, perhaps, but you get the idea.

This is the 25th anniversary of the series, which began as a 20-page pamphlet handed out to a few Europe-bound students in 1960. Today, "Let's Go: The Budget Guide to Europe" ($9.95), the largest volume in the series, tops 860 pages and sells 100,000 copies annually.

The books' usefulness, however, is not limited to students. Senior citizens are finding them helpful, as would anybody else watching pennies, or lire, carefully.

The continuing goal of these books is to provide "solid advice on how to eat, sleep and travel cheaply while really seeing the countries you visit." They do quite a good job of it. See FEARLESS, E8, Col. 1 FEARLESS, From E1

Forget luxury or even moderately priced hotels and restaurants. These guides direct you to youth hostels, dorms in colleges and religious institutions that take summer guests, private homes and inexpensive "pensions," or small guest houses.

As for food, they point you to inexpensive neighborhood cafes and cafeterias and also suggest cost-cutting tips for dining. To save money in Paris, for example, the traveler is advised to make a meal of gratine'e, or French onion soup with a bread and cheese crust, "about 20F $2 at any medium-sized cafe."

The emphasis is on the practical, how-to-cope type of advice: where it's safe to hitchhike; what student identification to carry for discounts; how to keep in touch with home. There's even a courage-boosting section on the rewards of traveling alone.

The basic volume is "Let's Go: Europe," which treats 33 countries, including -- and this is unusual for the standard guides to Europe -- the nations of East Europe and North Africa. Additionally, it and the other volumes in the series provide details on travel in the countryside as well as the major cities.

Also in the series: Britain; France; Italy and Tunisia; Greece, Cyprus and the Turkish Coast; Israel, Egypt and Jordan; Spain, Portugal and Morocco; California and the Pacific Northwest; the United States; and Mexico.

How do the authors find the bargains? "Our researchers," say the editors, "travel on budgets as limited as your own."

* FROMMER'S: Twenty-eight years ago, this popular series began with "Europe on $5 a Day." The daily rate has climbed, but not as much as you might expect. Author and publisher Arthur Frommer claims you can still tour "Europe on $25 a Day," the title of the 1985 edition ($10.95) of the original book.

The Frommer dollar-a-day guides also form a series devoted to budget travel -- focusing, too, on lodging and meals, but at a somewhat higher quality than found in the "Let's Go" guides. You move on to these books for a place to stay when dorm rooms no longer are a necessity.

The recommendations offered, says Frommer, "are designed for normal vacation living: tasty and filling restaurant meals, clean and comfortable hotel beds."

What you don't get for bargain prices, and Frommer states this emphatically, are private bathrooms. "Baths cost money," he writes, and anyway: "Insistence on a private bath will often make it impossible for you to stay in some of Europe's most delightful, and least expensive, budget lodgings . . . "

The rate of $25 he establishes is for "basic living costs": $12 to $14 per person for a double occupancy room; $2 for breakfast; $3.50 for lunch and $4 or $5 for dinner. Transportation, sightseeing and entertainment costs are extra.

The strength of the dollar-a-day guides is their listing of budget hotels and restaurants in 21 major cities of Europe, grouped sometimes according to price (Amsterdam) and sometimes according to location (London). They tend to be decent, unpretentious places that may do without a large lobby, coffee shop, room service and bellhops. Sometimes they occupy an upper floor of an office building.

He also offers a few "starvation budget" suggestions, and places for the "big splurge."

Typical of the lower-priced entries is the Pensione Sally, near the cathedral in the heart of old Florence. A double with breakfast is listed at$ 22: " . . . the situation of their hotel is magnificent, just a few feet from the Duomo. Stairs only; a fairly forbidding exterior; but large rooms."

A problem, however, is that so many people use Frommer, you might have difficulty finding a vacancy during the busy season.

Also available: "England and Scotland on $25 a Day"; Greece -- $25; Ireland -- $25; Israel -- $30 and $35; Scandinavia -- $25; Spain and Morocco -- $25; Australia -- $25; Hawaii -- $35; India -- $15 & $25; Mexico -- $20; New Zealand -- $20 & $25; New York -- $35; South America -- $25; Washington, D.C. -- $35. Each is written by a different author or pair of authors.

A companion series is Frommer's "Dollarwise" guides to lodging and meals "from budget to deluxe, with emphasis on medium-priced." Available are: Austria and Hungary; Egypt; England and Scotland; France; Germany; Italy; Portugal (Madeira and the Azores); Switzerland and Liechtenstein; Canada; Caribbean (Bermuda, Bahamas); California and Las Vegas; Florida; New England; the Northwest; the Southeast and New Orleans; the Southwest.

"It is the tourist who lives inexpensively on a European vacation," says Frommer, "who lies relaxed, who seeks out the European side of European life -- it's that person who enjoys a European trip."

* FIELDING'S: The principal audience for the Fielding's guides is the middle-income traveler, says a spokesman for the firm, which has been publishing guides for 38 years. So in "Fielding's Europe 1985" ($12.95), the "flagship" of the series, you find a much higher grade of hotels and restaurants, including some of the poshest in Europe.

The large volume -- 812 pages -- is basically a lodging and meals guide, awarding from one to five stars for particularly good establishments in the major cities of Western Europe. The lists are not especially long, but the places have been personally selected by Joseph Raff, long associated with Fielding Publications, and his wife Judy, both of whom write the book.

Each entry is accompanied by a concise but informative description that is useful in trying to decide where to stay and eat.

Along with the multi-starred places, however, are the Ruffs' "superb little sleepers," where, they write, "you might prefer to be anyway, because here is where you will find the real hometowners . . . "

For lower-priced travel, the Raffs have also produced "Economy Europe 1985," another hefty guide to the major cities. There's a fairly high standard to the listings here, too, so don't expect rock-bottom prices.

Three other Fielding Europe guides separately cover sightseeing, "selective shopping" and seeing Europe with children.

Also available: Bermuda and the Bahamas; the Caribbean; economy Caribbean; the Far East; an all-Asia budget guide; havens and hideaways; Mexico; Egypt and the archeological sites; a worldwide guide to cruises.

* FODOR'S: This series, originating almost a half-century ago, differs from the previous three in that the volumes tend to devote more space to touring information and historical background.

And like the "Let's Go" guides, they make a serious attempt to get you out of the big cities and into the countryside.

Of the six series in this column, these are the basic guides to begin planning where to go and what to see, particularly if it's your first trip to a new destination.

Fodor's has the advantage of 93 titles, an individual guide to practically every major tourist city and country, providing fairly detailed descriptions of sightseeing possibilities as well as basic historical detail.

In "Fodor's Eastern Europe 1985," for example, there are separate chapters of several pages each on Bulgaria's history, its arts and the food and drink.

Fodor's also provides restaurant and lodging suggestions -- from "inexpensive" to "super deluxe" -- although the lists aren't as extensive as, say, those in the Frommer's guides, and not nearly as much fun to read. The Frommer's writers can get quite enthusiastic about the charm of a favorite place; in Fodor's, the authors seem more subdued.

"Fodor's Europe 1985" ($13.95) is the everything-in-one-volume guide to 33 countries in 838 pages, but the real heart of the series is in the separate country and city guides. Also available are budget guides to several European countries, Japan and Mexico as well as a number of U.S. guides by city, state and region.

* BIRNBAUM'S: This series, like Fisher, represents an attempt to alter the A-to-Z format of many basic guides.

Birnbaum's "France 1985," the newest of the series ($11.95), provides good historical and cultural information about France and sightseeing, lodging and dining information for 16 cities.

But it goes beyond this standard information in two large sections, "Diversions" and "Directions."

The first is a very useful guide to a variety of vacations in France: skiing, tennis, fishing, great walks, boating the waterways, antique-hunting and touring the chateaus, among them.

Going abroad does not have to be an endless round of museum-viewing, and Birnbaum's series recognizes this. The "Diversions" section obviously is aimed at the increasing number of more-adventurous American travelers.

"Directions" is suggested touring itineraries to various sections of France. New visitors to a country, traveling on their own, can profit from this kind of guidance.

Also available: Europe; Great Britain and Ireland; Europe for Business Travelers; Canada; the Caribbean, Bermuda, the Bahamas; Disneyland; Walt Disney World; Hawaii; Mexico; South America; the United States.

* FISHER ANNOTATED TRAVEL GUIDES: The newest of the travel series, originating three years ago, the Fisher guides are aimed at "the experienced traveler" who is looking for "sophistication" in what to see and where to stay and eat. They are guidebooks more for the traveler with a comfortably filled wallet than the bargain-hunter.

The series is lively, and the unusual annotations, written in the margin in red, are often a delight. In "Paris" ($8.95), inscribed next to a description of the luxury Hotel Ritz, is the note: "This was President Nixon's favorite hotel." Next to the entry for the Bristol, another sumptuous hotel, the note reads: "And this was Kissinger's."

The books are very selective and very subjective -- awarding stars from one to five to rate the sights, the hotels and the restaurants. (The Ritz and the Bristol, by the way, both receive five stars.)

Most of the sightseeing information is found in a series of tours that include not only special historical and cultural attractions, but comfortable cafes and pubs along the way. In the "Paris" guide, the tours are a number of city walks "if you have only one day" or "if you have more time."

Also available: Europe; Britain; France; Germany; Greece; Italy; London; Portugal; Spain; Japan; the Bahamas; Bermuda; best of the Caribbean; Mexico; California and the West; Florida and the Southeast; Hawaii; Los Angeles; New England; Texas and the Southwest.