SAM WALTON: The Inside Story of America's Richest Man by Vance H. Trimble (E.P. Dutton: $19.95; 319 pp.) . Sam Walton, billionaire head of the Arkansas-based Wal-Mart retailing chain, readily admits to being aided by good luck--a luck that extends, it turns out, to his first biographer, who has written a virtual hagiography. "Sam Walton," by fellow Arkansan Vance Trimble, is unauthorized in name only: The real-life Walton no doubt resembles the inspiring, hard-working, just-folks executive depicted here, but the author's rose-colored perspective is a bit hard to take. Trimble, a Pulitzer Prize-winning reporter, often places problematic details in a positive light. He is little troubled, for example, by the tendency of new Wal-Marts to drive existing mom-and-pop stores out of business. It should be noted, too, that Walton never was America's richest man, Forbes magazine having failed for years to separate Sam's fortune from that of his family--a fact this book's publisher conveniently glosses over.