YOUNG MAN IN PARIS by John Weld (Academy Chicago: $13.95). Ah, to be in your 20s, in Paris, and moving in that circle of glamorous expatriates that peopled it between the two great wars. The whole thing fit John Weld, an ex-Hollywood stuntman and budding novelist, like a glove. Here he relives with gusto his idyllic five years on the Left Bank as a reporter for the Paris Herald, a job that threw him into intimate contact with everybody of note from Legs Diamond to Ernest Hemingway, F. Scott Fitzgerald, Ezra Pound and James Joyce. Far from a mere observer, young Weld was an enthusiastic co-conspirator in the high jinks of that brief, glittering period when so many of the English-speaking world's creative minds found the grass greener along the Seine. A delightful collection of fond, and sometimes ribald, memories.