[WORD ILLEGIBLE] 11-YEAR-old boys who live alone with zany writer-type fathers in cluttered, slightly misty New York apartments somethimes grow up to be J. D. Salinger characters. At least the potential's there with Sport Rocqua who more or less keeps home and father's sanity intact, all the while making sure that dad dosen't squander the remains of the royalty check on still another date.

It's a big order for the child to care for the man, but Sport has developed an innocent street smarts that only comes from years of eating canned spaghetti for dinner. Enter Kate, who only too knowingly arrives with a grocery bag full of steak and potatoes. How does Sport size her up? "He had long ago discovered that women who never intended to marry had very sharp, very pointed, very delicate and special shoes." Kate wore "just shoes."

Sport is understandably obessed with money, having had to scrimp so that Rocque might be free to pound away at the typewriter. As a wedding gift, he reverentially awards Kate his account ledgers. All's well until Sport's abeentee, detestable mother finally decides to lay claim on the child she traced for Europe, now that has due to inherit his grandmother's fortune. (One does wonder how a man as sensitive as Rocque ever married such a woman in the first place.)

Events could easily take a melodramatic twist were it not for the late Louise Fitzhugh's delicate eye for humor. Through a cascading chain of hilarious events, the Rocque establishment takes a beating. Sport's mother, oozing mink and martini with each shrill breadth, even goes so far as to kidnap her son. But ROcque and Kate get married anyway. Sport's money problems fall into place, and the first Mrs. Rocque decides that keeping her son is not worth the millions.

Sport's the last of a trilogy which began with Harriet the Spy and continued through The Long Secret. (Harriet, by the way, appears briefly in Sport, toting as always her black composition notebook.) A fresh and sophisticated story, it offers an understanding, clever look at the inconvenlence of having your dreams come true.