When he was a newspaper delivery boy in a New England town, Harry Barba rewarded himself each Saturday with a ''treat'' concocted by mixing leftovers from the family icebox and baking them in the oven of the wood- and coal-burning kitchen stove.
Two cookbooks have been indirect results of these boyhood culinary adventures. Barba and his wife, Marian, who live in upstate New York, published ''What's Cooking in Congress?'' in 1979 and a thicker ''What's Cooking in Congress?-II'' this summer (available in hard cover and paperback, Harian Creative Books, Hamilton Press, Castleton-on-Hudson, N.Y.).
As implied by the name and the subtitle, ''A Smorgasbord of Congressional Recipes,'' both books are filled with favorite recipes of prominent figures in Washington. In the newer version, metered lines of free verse telling about each president of the United States are scattered among directions for making President Reagan's fresh-peach ice cream, Sen. Edward M. Kennedy's Cape Cod fish chowder, and Sen. Barry Goldwater's Indian fry bread.
An interest in food and in cooking followed Harry Barba - a former professor with four degrees, a fiction writer, and a developer of writers' and educators' conferences - into adulthood. Somewhere along the line, he had hoped to open a restaurant called Originals Only, for which he would develop a new recipe each day, with emphasis on meats and fish prepared with fruits and fruit juices.
The dream was sidetracked, strangely, by the Watergate investigation. Barba, who possesses ''a healthy and abiding interest in the state of the nation,'' said he listened ''too intently to the Watergate hearings on radio and television. I got too involved, too caught up in it all.''
To relieve the tension and to ''present government in a different light,'' Barba and his wife, an artist and a schoolteacher, decided to compile a congressional cookbook.
They contacted friends who could put them in touch with members of Congress. ''We wrote letters, we telephoned, we talked with secretaries and aides,'' they recalled.
The second book went together more quickly than the first and with greater response. The Barbas plan to update the congressional cookbook every two years. Currently, they are reluctant to select favorite recipes because, Marian says, ''it would be both impolitic and impolite.'' Harry insists that recipes from President Reagan must head the list, so directions for a favorite vegetable salad, contributed by Henry Haller, chief chef at the White House, follow. Vegetable Salad 2 cups tiny green peas, cooked and drained 2 cups diced carrots, cooked and drained 2 cups diced celery 1 cup diced apple, peeled Salt and pepper to taste 1/2 to 1 cup mayonnaise
Combine cooled vegetables, celery, and diced apple in a large bowl. Sprinkle with a little salt and pepper and toss lightly with a fork. Add 1/2 cup mayonnaise, tossing lightly to distribute evenly, then add more mayonnaise as needed.
Chill 1 or 2 hours before serving, but not much longer.
An equal amount of diced, cooked celery root may be substituted for fresh celery if desired.