RKO went through so many management changes, from its birth in 1929 to its demise in 1957, that it never quite developed a screen personality of its own. Would any other studio have fired Orson Welles just a year after ''Citizen Kane''? Or launch Katharine Hepburn as a star and the plunge her into such a string of flops that she'd be labeled ''box-office poison''? The authors provide a ''production still'' and a neat paragraph for every RKO feature, distributing praise and blame in no uncertain terms. The result is a must for Hollywood buffs, and a handy guide to RKO pictures that still turn up on TV.