"Let's Go," the bargain travel guide series that for two decades has covered Europe like a backpacker's bedroll is finally going home.

An entirely new domestic guide "Let's Go: USA" (Dutton, 584 pp., paperback, $5.95) is now available, marking a new departure and direction for the guidebooks, which are produced by Harvard Student Agencies (Hsa) and written, researched and edited by Harvard and Radcliffe students.

The original guide, and still the most popular, is considered the Baedecker of bargain touring by many budget-minded travelers. The series -- the only one in the world prepared entirely by students -- also includes more detailed regional guides to Britain and Ireland, Italy and France. Another new guide covering Greece, Israel and Egypt will be published later this year.

"Let's Go: USA" is the sixth in the HSA stable, but it is a guidebook of a slightly different color. There are similarities between "Lets Go: USA" and its sister guides -- but a lot that is different, too. "

"The methodology is similar, but the approach had to be different," said C. Mark Battey, 22, the Harvard senior who is the current HSA publishing manager. For one thing, in the U.S.A. guide, comings as well as goings had to be taken into account.

The guide comes on the market as the balance of trans-Atlantic travel is undergoing a histoic shift: Last year, for the first time, more Europeans visited the United States than Americans traveled to Europe. About half the sales of "Let's Go: USA" are expected to be to foreigners.

So, this "Let's Go" tries to explain what makes the country tick as well as to list its highlights -- a tall order.

Editor and chief writer Kenneth Warren urges travelers to relax and approach this country the way a jazz musician tackles a tune.

"The word is improvisation. It's the heart of jazz. It's also the heart of travel: No rigid schedule can equal the sense of discovery you get when you follow an impulse and take an unplanned route through a town. You'll see the things which never appear in chamber of commerce brochures and you'll find the places which aren't included in any tour."

Chamber of commerce brochures proved something of a problem to the HSA staff when research first began on "Let's Go: USA." "We wrote to chamber of commerces all over the country -- and got back a ton of glossy brochures," recalled Dan DelVecchio, HSA general manager. To HSA's surprise, the brochures weren't all that helpful since, in sharp contrast to Western Europe, there was no qualitative rating of hotels, restaurants or sights. To a chamber of commerce, every establishment that pays its dues is wonderful.

In the end, most of the information in "Let's Go: USA" came from the notes of the 32 student researchers who fanned out around the country, eating in cafeterias and diners, sleeping in guesthouses, youth hostels and campgrounds; and traveling by car, bus, train and thumb.

To ensure a standard approach, all researchers carried a questionnaire similar to the one used for "Let's Go: Europe," which asked for the best, the worst, the cheapest, etc. To encourage a budget approach, each researcher received an expense stipend of $300 to $1,600 -- those who stayed well within it could pocket the difference. Researchers who got their copy in on time, and were unusually enterprising, were also eligible for a bonus of up to $300.

"Our researchers are the best budget travelers in the world," said Battey, "they all have iron stomachs." Sections on major cities were done by people who knew them well -- usually natives -- to get an insider's approach. "New York was done by a real New Yorker and we took the safety of the traveler into consideration," Battey noted. In non-metropolitan areas a conscientious enthusiasm was considered an acceptable substitute for local knowledge.

Emphasis was put on train and bus travel both for reasons of economy and because that's the way foreigners are used to traveling. "The train has played an almost mythic role in the growth of the United States. It has been a symbol of power and expansion in a nation that most worships these two qualities," the guide informs its readers. And then adds: "But, in recent years Amtrak, the nation's major train line, has fallen upon hard times . . . Still, train travel is attractive."

Bus travel was even more attractive to HSA researchers, Battey said: "It's cheap and also convenient. The Rocky Mountain section was researched entirely by bus and it's one of the stronger sections. "Let's Go" divides the United States into nine geographical regions, said Battey, "because that's the way people visit the U.S."

Editor Warren felt the United States is best appreciated as the sum of its parts: "The U.S.A. is a surprising concoction of elixirs, potions, powders and dyes from all over the world. Everything is vigorously blended; but almost magically, much remains unmixed, waiting for the traveler to come along and taste the individual ingredients."

Oly 32 states and the District of Columbia are actually covered in the guide. A box entitled: "Where's My Hometown?" explains: "This year's 'Let's Go: USA' is a new venture, and the difficulties which attend any new project have made it impossible for us to include each state in the U.S.A. The omissions do not mean that we feel that these areas are unimportant, but that schedules and deadlines did not leave us time to adequately research them."

That is small consolation to the citizens of Vermont, North Dakota, Mississippi and the other 15 states arbitrarily dismissed from the union. HSA promises that more states will be added in subsequent editions of "Let's Go: USA."

"Vermont will probably go in the next edition," Battey said. "There is a really strong Vermont lobby -- but it will be a while before we get to all 50 states."

The guide is not definitive, but is certainly detailed giving meal prices in listed restaurants down to the penny. A one-pound lobster at Cap'n Newick's in South Portland, Maine, is $5.95, for instance, while $6.75 will get you filet of sole en papilote at Tarantino's on San Francisco's Fishermen's Wharf.

However, today's inflation makes efforts to give accurate prices almost an exercise in futility. The U.S.A. guide carries a prefatory disclaimer saying that pices were accurate when they were researched, but because of inflation travelers should expect increases of 10-30 percent.

The "Let's Go" series began humbly in 1960 as a 12-page mimeographed booklet of travel tips distributed to passengers on HSA's Europe-bound charter flights. Over the years it evolved into a full-fledged student guide -- sometimes known as "The Hippies' Handbook" -- but older readers also found the guide helpful and the word "student" was dropped from the subtitle and replaced by "budget." In 1970, HSA went into partnership with the E. P. Dutton publishing company.

Some 85,000 copies of "Let's Go: Europe" are sold annually. "Let's Go: USA" is expected to sell more than 50,000 copies -- and there is already talk of a separate European edition.

Royalties go to HSA, a non-profit organization that uses the income to create part-time employment for Harvard and Radcliffe students. So, chances are this summer there will be even more student researchers roaming the country looking for the worst, the best and the cheapest.