With dozens of "vegetarian" cookbooks flooding the marketplace, it's a pleasure to find two books on vegetables by authors who know how they should be handled.

"A Celebration of Vegetables" by Robert Ackart (Atheneum, $10.95) combines vegetable recipes in menus and includes desserts.

Vegetables are arranged under menus, the menus grouped according to seasons. At the beginning of each recipe the kind of information cooks like to know before they start is offered: preparation and cooking time, whether or not the recipe can be made ahead of time and if it can be doubled.

On the other hand, "Out of the Garden Into the Kitchen" by Beryl Marton (David McKay, $12.95), is arranged by vegetables. Each chapter includes nutrient information along with gardening directions and basic cooking preparations.

Here is a recipe from Marton's book, followed by two from Ackart's. SQUASH AND ONION SAUTE (4 servings) 4 yellow squash or zucchini, or two 10-ounce packages frozen sliced squash 2 onions, sliced 4 tablespoons butter Salt and pepper to taste 1 cup sour cream 1/2 cup grated parmesan cheese

Scrub and slice squash. Saute onion in butter until limp. Add squash and saute until tender. Season with salt and pepper. Stir in sour cream and heat, but do not boil. Sprinkle with cheese and serve at once. EGGPLANT AND ZUCCHINI IN CUSTARD (6 servings) Olive oil 1 large eggplant, unpeeled and cut in 1/4-inch slices 4 medium zucchini, cut into 1/4-inch slices 1/2 cup finely chopped parsley 1 teaspoon basil Salt Freshly ground pepper 4 medium tomatoes, peeled, seeded and chopped 1 large onion, peeled and chopped fine 1 cup light cream or half-and-half 1 egg 1 egg 1/2 teaspoon cinnamon 1 egg yolk (optional) 1/2 pound mozzarella cheese, thin sliced

In olive oil, saute the eggplant on both sides. Remove it to absorbent paper. Repeat the process with the zucchini. Cut the eggplant slices into quarters. In a lightly oiled ovenproof serving dish, arrange the eggplant in an even layer. Over it, sprinkle one-half the parsley, basil and salt and pepper to taste. Repeat with zucchini. Over the top, spread evenly first the tomatoes, then the onion.

In mixing bowl, beat together the cream, egg and cinnamon. (If firmer custard is desired, add 1 egg yolk.) (At this point you may stop and continue later.) Over the contents of the baking dish, pour the prepared custard mixture. Garnish the dish with mozzarella slices. Bake the loaf in the upper third of the oven at 350 degrees for 40 minutes, or until the cheese is set. CASSEROLE OF GREEN BEANS (6 servings) 1 1/2 pounds green beans, stems removed, halved, and rinsed in cold water 4 tablespoons butter 4 tablespoons flour 2 1/2 cups milk 1 1/2 cups grated milk Cheddar cheese Grating of nutmeg Salt Freshly ground black pepper 1 cup bread crumbs, toasted in 2 tablespoons butter (optional) Paprika

In soup kettle, bring to a rolling boil several quarts of salted water. Into it, plunge the beans; leave them uncovered. After the water has returned to the boil, cook the beans for 12 minutes, or until they are just crisp-tender. (The age of the beans will determine their cooking time.) Refresh them in cold water; drain and reserve them.

In a large saucepan, heat the butter and in it, over gentle heat, cook the flour for a few minutes, stirring. Gradually add the milk, stirring constantly until the mixture is thickened and smooth. Away from the heat, add the cheese, stirring until it is melted. Season the sauce to taste. Into the sauce gently fold the beans. Arrange the mixture in a buttered ovenproof serving dish. Prepare the bread crumbs. (At this point you may stop and continue later.) Spread the bread crumbs evenly over the beans. Add a sprinkling of paprika. Bake the beans at 350 degrees for 15 minutes, or until the dish is well heated and the crumbs begin to brown.