Yogurt - fast becoming a staple in many American kitchens

Americans, tired of winter but not in the right part of the world to enjoy what a traveling friend once described as ''the full moon and the Bay of Bengal rising up out of a purple mist,'' can anticipate warm summer weather by mixing up a soothing glass of lassi or any of a number of yogurt-based drinks.

Lassi, a drink from Nepal, is watered yogurt or buttermilk. If you whip it up with banana or any other fruit, it is a cool, soothing drink.

The United States, slow to learn about this ancient food, has suddenly become a nation of yogurt eaters: Some 1.3 billion eight-ounce containers were sold in this country in 1980, according to a recent New York Times article.

For those who want to learn more about yogurt, Shaun Nelson-Henrick has written, albeit in a very breezy style, a surprisingly interesting book, ''The Complete Book of Yogurt'' (Macmillan, $16.95; Collier Books, softcover, $6.95).

The book has over 350 recipes that use yogurt for basic foods from soups to salads, including breads and muffins, pancakes, beverages, and frozen yogurt.

The recipes involve more than just substituting yogurt for sour cream or mixing yogurt and brown sugar with green grapes for a tried-and-true hot-weather dessert.

There is a wonderful combination of onions sauteed with spices and mixed with yogurt and diced, cooked potatoes, and there are recipes for fish poached or baked in yogurt.

Yogurt can be mixed with eggs before scrambling to make them lighter, and is suggested as a topping for peaches or apples under a pie crust.

But Mrs. Nelson-Henrick does more than write recipes for yogurt. The first half of the book tells us all we want to know about yogurt, and probably more.

She has talked to the Dannon people, who started producing 200 bottles of plain yogurt a day in the Bronx in the 1940s. She points out that Breyer's, Columbo, Continental, and Maya are the names to seek out for whole-milk yogurt, while those wanting low-fat yogurts prefer such names as Dannon, Sealtest, and Light 'n Lively.

There are detailed directions on how to make yogurt at home - with or without a yogurtmaker - as well as comparisons of seven yogurtmakers.

Directions tell how to make yogurt cheese, how to use yogurt as a sourdough starter, how to make and where to buy kefir (a cultured milk drink), and how to make frozen yogurt.

Here is a recipe for lassi: Lassi 1 /2 cup plain yogurt 2 cups cold water Ice cubes

Place yogurt in a blender and blend for 10 seconds. Add water, ice, and flavoring, if desired. Blend briskly for 1 minute. Serve in a tall glass. Yield: two 10-ounce servings.

Variations: For salty lassi, add salt and pepper to taste. For sweet lassi, add 3 drops of rose water and sugar to taste. For masala lassi, add 1/4 teaspoon of cumin powder, 2 leaves of coriander, and salt to taste.

One old Pakistani hand suggests the addition of soda water to salty lassi for a particularly cooling drink.