A VEGETARIAN diet is fairly easy to follow, says Cheri Jones, assistant professor of nutrition at Loma Linda University, especially with the help of the dozens of vegetarian cookbooks available these days.

She recommends reducing the fat content of most of the recipes, however, by decreasing the amount of butter and oil required for frying or tenderizing certain foods.

"Laurel's Kitchen," a book published in 1976 by Laurel Robertson, Carol Flinders and Bronwen Godfrey, followed "Diet for a Small Planet" by Frances Moore Lappe' as a sound vegetarian handbook. This book, recommended by such nutrition authorities as Jean Mayer (president of Tufts University), is included on a reference list circulated by the American Dietetics Association.

Carol Flinders, mother of a 2-year-old and a syndicated columnist, cites two or three weaknesses in the common ovo-lacto vegetarian diet. First, she says, some foods can be abused on the diet, notably dried fruits, nuts and honey. Dried fruits, although nutritious, are concentrated sources of sugar and calories. Nuts are very high in fat. And honey, like sugar, is a concentrated sweetener with very little value other than calories.

Second, she says, parents must set a proper example, eating a variety of fresh, whole foods in moderation.

And last, she says, "Everybody raising kids should be aware of the nutritional importance of exercise." Recent research shows that exercise may improve nutrient absorption. Studies also show that young people are eating fewer calories but may be overweight because they don't move enough. "When they sit in front of the television all day, they don't get hungry enough to eat the things they need," explained Flinders.

Her book, "Laurel's Kitchen," outlines basic nutrition requirements, gives recipes and nutrition tables, and interprets the basic four food groups in ways suitable for lacto-ovo vegetarians (who eat dairy products and eggs) and vegans (who eat no animal products).

Beyond that, choosing a vegetarian cookbook is like choosing any cookbook. You have to find one you're comfortable with.

A few mentioned in and informal survey of vegetarians are:

* "The Vegetarian Epicure," editions I and II, by Anna Thomas (Vol. I, $6.95 paper, $16.95, hardcover; Vol. II, $6.95 paper, $12.95 hardcover; Alfred Knopf). Thomas gives a sophisticated approach to the diet. Some of the recipes take a long time to prepare, but all are consistently delicious. There are menu ideas and an excellent index.

* "The Moosewood Cookbook," by Mollie Katzen ($9.95, Ten Speed Press). Popularity grows for this book, which has received very little promotion. The recipes are consistently good, many are easy to prepare, but they are often very high in calories. Recipes are easily modified, however. The mushroom strudel, which calls for cream cheese and sour cream, tastes wonderful when prepared with tofu and yogurt.

* "Cashews and Lentils, Apples and Oats," by Diana Dalsass, ($7.95, Contemporary Books). Arlington vegetarian Carol Gigliotti calls this a good "basic" cookbook "for someone who's just starting."

* "A Celebration of Vegetables," by Robert Ackart, ($4.95 in paper, Atheneum) uses menus to guide the cook through four seasons of vegetarian meals.

For recipes, turn to any cookbook with which you are comfortable, and don't overlook all-purpose cookbooks that contain vegetarian recipes or standard recipes that are easily converted. Approach any nutrition advice with extreme caution, however, using the U.S. Dietary Guidelines as a guide (eat a variety of foods; maintain weight; avoid too much fat, saturated fat and cholesterol; eat foods with adequate starch and fiber; avoid too much sugar and sodium; if you drink alcohol, do so in moderation).

LASAGNA AL FORNO (4 to 6 servings) About 9 whole-wheat lasagna noodles (substitute regular noodles, if necessary) 1 bunch spinach or swiss chard, or 1 package frozen spinach 3 cups tomato sauce (recipe follows) 1/2 cup chopped, toasted walnuts or almonds 1 cup low-fat cottage cheese 1/4 cup grated parmesan cheese 4 to 8 ounces grated mozzarella cheese

Cook noodles in boiling, salted water until tender. Drain. Wash and dry spinach and chop into bite-sized pieces (or thaw and drain frozen chopped spinach, and add it to tomato sauce). Spread 3/4 cup of sauce in bottom of 8-by-8-inch baking pan. Place 3 of the noodles on the sauce (trimming them, if necessary; they should cover the sauce). Cover with 1/3 of the fresh spinach, and top with 1/4 of the nuts, 1/4 cup cottage cheese and 1 tablespoon parmesan. Sprinkle with mozzarella. Repeat layers twice. Spread sauce over the top and sprinkle with remaining nuts and cheese. Bake at 350 degrees for 40 minutes. Allow to stand 10 minutes before cutting.

TOMATO SAUCE (2 to 3 cups) 1/2 onion, chopped 2 cloves garlic 2 tablespoons oil 1 small carrot, grated 2 tablespoons chopped green pepper 1 bay leaf 1 teaspoon oregano 1 teaspoon basil 1/2 teaspoon thyme 2 tablespoons chopped fresh parsley 2 cups tomatoes, fresh or canned 6-ounce can tomato paste 1/2 teaspoon salt 1/4 teaspoon brown sugar 1/8 teaspoon freshly ground black pepper

Saute onion and garlic in oil until onion is soft. Add carrot, green pepper, bay leaf and herbs. Stir well. Add tomatoes, tomato paste and seasonings. Simmer 30 minutes, uncovered. Remove bay leaf. This recipe doubles and triples; freezes well. From "Laurel's Kitchen," by Laurel Robertson, Carol Flinders and Bronwen Godfrey

TOFU PATTIES (2 dozen)

This dish might sound inordinately arcane, but Carol Gigliotti insists her 2-year old loves this dish, and that the patties are better the second day. She varies the recipe by blending a pound of drained tofu with bread crumbs, an egg and 3 tablespoons of nutritional yeast and flavors them with garlic, rosemary or dill and a little tamari (soy sauce). The mixture should hold the patty shape and may be fried in a little vegetable oil. The following recipe, with explicit directions, might make the uninitiated tofu cook a little more comfortable. 1/2 onion 1 stalk celery, finely chopped 1/2 green pepper, finely chopped 2 tablespoons oil 20 ounces (more or less) tofu 1 egg, beaten 2 tablespoon whole-wheat flour 1/2 teaspoon salt 2 tablespoons soy sauce 2 teaspoons curry powder or 1/2 cup grated cheddar cheese Wheat germ, cornmeal or sesame seeds

Chop onion, celery and green pepper. Saute in oil until soft. Drain tofu, then mash it with fork and mix in egg, flour, salt and soy sauce. Add vegetables and either the curry powder or the cheese. Form into small patties and roll them in wheat germ. Brown on lightly greased griddle or skillet, or bake in 350-degree oven until heated through and browned, about 20 minutes. From "Laurel's Kitchen," by Laurel Robertson, Carol Flinders and Bronwen Godfrey

SPINACH AND ORANGE TART (4 to 6 servings) 1 cup evaporated milk 3 egg yolks 1/4 cup sugar Pinch salt 1 1/2 pounds fresh spinach, blanched (substitute 2 10-ounce packages frozen) Grated rind 1 small lemon 1 teaspoon vanilla 9-inch unbaked pastry shell 1 cup orange marmalade, melted

In the top of a double boiler, using a rotary beater, blend the milk, egg yolks, sugar and salt. Over gently boiling water, cook the custard, stirring constantly, until it coats the spoon. Add the spinach, lemon rind and vanilla, stirring until well blended.

Into the pastry shell, spoon an even layer of 1/3 of the spinach mixture. Over it spread 1/2 of the marmalade. Repeat. Finish with a layer of the remaining spinach mixture. Bake at 450 degrees for 20 to 30 minutes, or until the custard is set and the pastry is golden. Allow the tart to cool and serve it at room temperature. From "A Celebration of Vegetables," by Robert Ackart

POTATO PUFF (6 servings) 1 cup milk 3 egg yolks 1 medium onion, chopped coarsely 1 cup grated cheddar cheese 3 medium potatoes, peeled and chopped coarsely 1 1/2 teaspoons salt 1/4 teaspoon pepper 1/3 cup finely chopped parsley 1 small green pepper, seeded and diced fine 3 eggs whites

In an electric blender, combine the first seven ingredients and, on medium speed, whirl them until the mixure is smooth. Transfer it to a mixing bowl. Stir in parsley and pepper. Beat egg whites until stiff, but not dry. Fold the egg whites into the potato mixture. Spoon the batter into a buttered 2-quart souffle' dish. Bake the potato puff at 350 degrees for 1 hour, or until it is well risen and golden. Serve at once. From "A Celebration of Vegetables," by Robert Ackart

ORIENTAL SANDWICH SPREAD (4 sandwiches)

This uniquely flavored sandwich spread combines the smoothness of cream cheese with the nutrition of peanut butter and sesame seeds. Soy sauce and ginger give it zip. The spread is also ideal appetizer fare stuffed into celery stalks or raw mushroom caps.$ 3-ounce package cream cheese, softened 1/4 cup peanut butter 2 tablespoons sesame seeds, toasted in an ungreased skillet until golden 1 tablespoon yogurt 1 teaspoon soy sauce 1/8 teaspoon ginger

Place all ingredients in a bowl and mix well. Store, covered, in the refrigerator. From "Cashews and Lentils, Apples and Oats," by Diana Dalsass