Onetime-slapsie Maxie Shulman is slapsie no more. His Rally Round the Flag, Boys! (Doubleday, $3.50), though a funny book, funnier than many being written these days, does not read like a product of the same antic pen that gave us Sleep till Noon and Barefoot Boy with Cheek, those flights of fantastic fooling that made the name of Shulman synonymous with split sides. Rally is milder, much milder: slick, safe-and-sane, earth-bound, commercial, Good Housekeeping humor, eminently suitable for filming (20th Century has already bought it for 200 Gs). A sort of Exurbanites-with-plot, it limns the adventures, amorous and otherwise, of various fauna residing in Putnam's Landing, Connecticut, and tells what happens when an Army Nike base invades such a social structure. What happens? Take your pick: some infidelity, some GI clowning, some teenage shenanigans, some unity, some disunity and a happily-ever-after ending. A cruel critic would call it Thin. A nice guy like us, gin-buck in hand, criticism numbed by the opium of summer, would call it Good Clean Fun. But we don't think you'll miss much if you decide to wait for the movie.