Trillin spices his food with humor; Third Helpings, by Calvin Trillin. New Haven and New York:Ticknor & Fields. 184 pp. $12.95.

There's something refreshing and reassuring about a book by Calvin Trillin. Unlike those who write of food and restaurants in tones more appropriate to contemplations of the Mona Lisa or the Bay of Naples at sunset, Trillin writes with wit, humor, and a commendable sense of proportion. He knows that a delectable delicacy can just as easily be found at a street vendor's corner stand as in those places where the wine lists are as heavy as dictionaries, kiwi fruit has replaced the parsley on the side, and (in deference to the rigid demands of nouvelle cuisine) everything tastes of raspberry vinegar.

In ''Third Helpings,'' his latest book, Trillin sets out to defend his eating habits. His wife, Alice, has charged that he is in danger of becoming a ''food crazy,'' the kind of person who seeks out the best burrito in East Los Angeles. He realizes that only serious research, of the kind that in more cultivated times was done by educated gentlemen interested in knowledge for its own sake, would prevent her making comments like ''You're making an absolute pig of yourself.'' And to demonstrate the seriousness of his intentions, Trillin, in the first essay, describes his attempts to replace the Thanksgiving turkey with Spaghetti Carbonara. After all, he argues, no one knows that the Pilgrims really ate turkey at that first dinner. Not only would Spaghetti Carbonara be a belated tribute to Christopher Columbus, but Trillin just happens to love the dish, and regards turkey ''as something college dormitories use to punish students for hanging round on Sunday.''

His research is wide and wonderfully varied. He researches the famed Buffalo chicken wings, which have earned the much-maligned city of Buffalo a growing culinary reputation and which even Craig Claiborne has pronounced excellent. He looks into the difficulties of reading the menu in Chinese restaurants, where one watches enviously as Chinese customers ''enjoy some succulent marvel the management has not bothered to translate into English.''

An Italian street festival in Greenwich Village is an opportunity to discover the perfect sausage sandwich, which must be eaten while standing up to be enjoyed. No purchase can be made until all the stands have been inspected for green peppers, onions, and the roll that will most perfectly complement the sausage. His two daughters tag behind reminding him that it is a school night.

Trillin investigates the fried-chicken wars of Kansas, the perils of professional food-tasting, clambakes, food in Japan and Hong Kong, and a catfish festival in Florida. He is tireless in his pursuit, although his wife and daughters occasionally flag.

Some of the material has appeared elsewhere, but with the addition of new pieces it's a splendid collection to dip into and savor.