While the reader who adores Truman Capote's Breakfast at Tiffany's or In Cold Blood hopes to glean something about craft or inspiration from this collection of letters, the gossipeuse hopes to discover the vintage dish of a noted social climber and party host. It's tough to say who will be more disappointed. The writerly wisdom is as thin (''write about what you know'') as the put-downs are weak: Orson Welles is ''somewhat pathetic''; Spain is ''ghastly''; James Baldwin's fiction is ''of a balls-aching boredom.'' In between, there are weather reports, reproaches, flattery, and updates on tomato preserves. Too Brief a Treat is a recipe for balls-aching boredom.