BOOK SALES are supposed to be off 10 to 15 percent, but you'd never know it to count the cookbooks that have been published this fall.

Perhaps cookbooks are exempt from this decline. With the exception of a few being published, most of the newest books are so specialized they will appeal only to a narrow segment of the cooking public. But that also means there is something for everyone.

If you don't know the first thing about cooking, you'll find help in this latest crop. If you are in your nouvelle cuisine phase, you have not been forgotten. If you have joined the march to vegetariansim, you have several choices.

Do you worry about additives? Is someone in your on a special diet? Are you into nostalgia or perhaps a return to a time when cooking meant spending most of the day at it?

The publishing business has just what you need, some of it quite good.

For nouvelle cuisine fans, of course, there is Michel Guerard's Cuisine Gourmande (Morrow, $14.95.)

Guerard is the three-star French chef who brought the phase cuisine minceur to America and thereby permanently confused a number of people. Cusine minceur is diet cooking; nouvelle cuisine is not. Guerard's gourmande cuisine is his way of describing this new cooking. It is simplified, light when compared to French classical cooking, and depends on the freshness of the ingredients and a minimum amount of cooking. It is not, however, calorie conscious, which Guerard points out in a discussion about the use of flour to thicken sauces: "This is done less and less today," he says. "Nevertheless, handled with finesse, binding with flour is better for you than the enrichment of sauces with outlandish quantities of butter and reduced cream. However," he continues, "my personal view about sauces leads me not to make much use of either of these methods, even in cuisine gourmande . . ."

Guerard and other followers of new cooking like to thicken with vegetable purees. "These purees . . . have a high vitamin content and the vitamins are all the more easily absorbed because the cellulose in the vegetable is broken down in the pureeing process," he writes.

What Guerard's translator's have concerned themselves with is the difficulty home cooks might have trying to purchase or make some of the ingredients for the recipes. So they offer a number of alternatives.

If shallots are not avilable, scallions or even onion, though quite different, can be used instead.

To simplify things, Michel Guerard uses only one kind of stock in the gourmande recipes in this book -- chicken stock.

Two recipes are given: one from scratch and one for improving the quality of canned chicken stock.

Some purists may consider these substitutions to be cop-outs. Others would describe them as realistic. The book is a mixture of both, ranging from what only the most dedicated hobbyist would attempt to recipes a moderately accomplished cook could produce. To make it a bit easier, each recipe, includes a list of utensils needed.

Noted for its food pictures ever since the Time-Life cookbook series made its debut in the '60s, the current series entitled The Good Cook, will not be a disappointment. The how-tos in the first four volumes of the series are profusely and clearly illustrated. The original versions of the book were created in London, but the recipes, which have been borrowed from other cookbooks, are mainly American. To the extent that the original cookbooks offer good, accurate recipes, so does The Good Cook series. They run the gamut from homespun to classical French, from Mexican to Maltese, from very old to very new.

The chief consultant for the series is the well known American expatriate cook and cookbook author, Richard Olney.

Each of the books -- on vegetables, poultry, fish and beef and veal -- sells for $9.95.

Those who like to read cookbooks as well as eat from them will enjoy From My Mother's Kitchen by Mimi Sheraton (Harper & Row, $12.95).

Shearton, who grew up in Brooklyn, comes from a family whose whole life was wrapped up in food. Her father was a produce buyer; her mother, Beatrice Solomon, and grandmother, from whom Sheraton learned her craft, were excellent home cooks. All of them are responsible for her high standards, standards which she brings to The New York Times as food and restaurant critic.

So it is comforting to know that Beatrice Solomon's cooking was "done with the worst sorts of utensils" which "made a mockery of all recipes calling for well-insulated heavy pots and enameled exteriors."

And to know that Solomn had a "distinct proclivity for setting fire to the kitchen."

Sheraton's family recipes are a mixture of Old World Jewish and New World Gentile. Her mother did not keep a kosher home, so mixed in with the gefilte fish, challah and vareniki (boiled fruit-filled dumplings) are directions for steamed clams, baked lobster and fried oysters.

Sheraton's mother was not very happy about the book. She never measured anything and hated having her daughter, measuring spoons and cups in hand, following after her as she cooked. Solomon, who died just as the book was going to press, begged her daughter to drop the project. Sheraton recounts their discussion in the book's introduction:

"I'll pay you not to do the book,' she said.

"How much?" I asked, certain I would be the first writer to be subsidized for not producing.

"Fifty dollars,' she answered, obviously her estimation of my worth on the open market."

Followers of The Husband's Cookbook in these pages will be pleased to know that the author, Mike Mcgrady, has put his 52 menus between hardcovers (Lippincott, $9.95).

McGrady's menus and humorous how-to for the most inept of cooks is the result of the year he spent as a househusband, trading roles with his wife.

While the publisher describes it as a cookbook "primarily for men," a more accurate -- and less sexist -- description would be as a book for novices.

Simca, who along with Julia Child and Louise Bertholle brought French cooking to Americans, has discovered the food processor. In her latest book, written with Michael James, she also explores the use of such very American ingredients as bourbon and maple syrup.

New Menus from Simea's Cuisine (Harcourt Brace Jovanovich, $12.95) offers a couple of enticing vegetable purees, which seem to have become the hallmark of nouvelle cuisine , an intriguing mayonnaise made without eggs and a highly individual approach not only to recipes but menus.

There are 22 menus and for those who do as much preparation as possible with a food processor, this is a book to add to the collection.

The consumption of vegetables is on the rise and so is the appearance of vegetable cookbooks. They are not to be confused with vegetarian books, which are also appearing in greater numbers.

Jane Grigsons's Vegetable Cookbook, the work of an English woman writing for the English market, has just been published in this country (Atheneum, $19.95). Grigson, a well-resepcted food writer, takes note of the difference between English and American cooking in her introduction: " . . . I hope," she writes "you will not find 'the Vegetable Book' as alien as you might have expected."

And in many ways she is right. But the American publisher should have taken the trouble to make the changes which would have assured a truly American version of what has obviously been a very successful book in England.

Americans do not call eggplant aubergine unless they are speaking French. But the book does. Nor do we call zucchini courgette .

While all of the European measures in litres and grams are translated into American pounds and pints -- as, for instance, $150 ml (1/4 pt) wine or cider vinegar" -- for this market the European measure should have been put in parenthesis.

Nor is it easy for cooks here to know what "plain or strong flour" is, or for that matter Cox's Orange Pippins (apples, as it turns out).

For those willing to put up with these inconveniences, however, the book offers an extensive variety of vegetable dishes, some quite different.

Vegetarian cookbooks are no longer directed to the already converted: Now they want to attract those who might be tempted, but think (A) its too time consuming; (B) its too fattening; (C) it doesn't taste good.

The Vegetarian Feast, by Martha Rose Shulman (Harper & Row, $12.95), and the paperback The Quick & Easy Vegetarian Cookbook by Ruth Ann and William Manners (M. Evans, $4.95) set out to convince the unconvinced.

For those who think vegetarianism means only beans and rice and sprouts and tofu, the recipes will come as a surprise. Both books are a mixture of what one traditionally expects to find in vegetarian cooking and what, if you gave it any thought, fits there just as readily. The only things missing are meat, fish and poultry.

Admirers of Laurel's Kitchen, an excellent vegetarian cookbook of several years ago, may enjoy her 1980 calender with 60 recipes that include her incomparable oatmeal cookie recipe.

Copies of the calendar are available for $5.45 including postage from: Nilgiri Press, Box 477, Petaluma, Calif. 94952.

Some cookbooks are worth writing about twice. Jeanne Lesem's The Pleasures of Preserving & Pickling is one of them. Published four years ago in hardcover, it is now available in paperback from Sunridge Press for $4.95.

Lesem, food and family-living editor for United Press International, proves that foods can be "put up" almost anywhere, if the interest is there. She does all her picking and preserving in an 8-by-9-foot Manhattan apartment kitchen.

In addition to all, the essential information for how to do it, Lesem has some delightful recipes that go well beyond the standard dills and jellies: apricot cordials, lemon-flavored mustard, pepper sherry, and one that makes most old-time cooks do a double take -- frozen sliced sweet dill pickles.

The woman who was one of the first to warn us about the hazards in our food supply in "The Mirage of Safety" is still in there fighting. Beatrice Trum Hunter's The Great Nutrition Robbery. (Scribner Library, $4.95) has moved on from safety to nutritional hazards. She explores what happens to the value of food when imitation ingredients are substituted for the real thing.

Discussing margarine versus butter, Hunter comments on animal feeding studies. If they are applicable to humans, she writes, "it means that margarine, which contains a high level of trans fatty acids, is more likely to cause atherosclerosis than cholesterol-rich animal fats such as butter or cholesterol-rich animal foods such as eggs."

In the orange juice versus orange drink war, Hunter notes the differences: "Real orange juice contains fractions of the Vitamin b complex: niacin and thiamine, which may be totally absent in synthetics. Orange juice, an excellent source of potassium, contains more than four times as much potassium as the synthetic fruit drinks. Doubtless there are many other important nutrients, including other vitamins, minerals, trace minerals, bioflavonoids, and fiber, that are well balanced and available in real orange juice but not yet listed in nutrition charts."

In light of the not-so-slow but very steady move toward highly proceeded, fabricated, laboratory-created foods, Hunter's book is a helpful guide through the maze.

For those who thinks the Colonel's fried chicken is better than anything they can make at home, that McDonald's special sauce is the ultimate for hamburgers, Gloria Pitzer's latest book is a must.

The woman who first told American cooks how to recreate the food they liked in fast-food restaurants has published her fourth Secret Recipes cookbook. It contains 500 recipes, not all of them from fastfood restaurants by any means.

Pitzer, who also sends out a recipe magazine to her subscribers each month, has branched out: ". . . we are going in a more practical direction than when we emphasized fast foods," she says.

Her latest book is available for $5 from Secret Recipes, Box 152, St. Clair, Mich. 48079.

Parents of hyperactive children who want to treat them through dietary management instead of drugs will find The Feingold Cookbook for Hyperactive Children, by Dr. Benjamin Feingold and his wife Helene (Random House, $5.95 $, of enormous help.

The Feingold Diet, which eliminates certain food additives and foods which are natural sources of salicylates, is named for the allergist who first expounded the theory about the relationship between hyperactivity and diet. The theory is extremely controversial in medical circles, but parents who have had success treating their hyperacitve children with the diet don't find it controversial at all.

The American Heart Association Cookbook is in its third edition (McKay, $12.95). It concentrates on the fat content of the diet, though it makes reference to the need for a reduction in salt intake and weight control. Heavy emphasis is place on the importance or reducing cholesterol and saturated fat consumpton. The book recommends that "the total fat in the diet should comprise no more than 30 to 35 percent of total calories." Many nutritionists have gone below that range, recommending no more than 30 percent fat at the most.