PEOPLE WHO KEEP SECRETS don't necessarily have secrets worth keeping. Often, it's simply a desire to whet the listener's appetite for the information, without there being too much need for mystery. At least that's how we're regarding the announcement from Macmillan that they've acquired the Alexander Haig book being peddled around New York by Owen Laster and Norman Brokaw of the William Morris Agency. Spring 1984 is the expected publication date. No title as yet--that's okay. But, who's the editor? "Book Report" inquired of the Macmillan spokesperson. That I can't tell you yet, she replied. Does he/she have to undergo a national security check first, or is a crash course in Haigspeak holding up the release of the lucky employe's name? . . . Joan Kahn turn, turn, turns again. Now, it's up at St. Martin's after a brief stop at Dutton. Before that was Ticknor & Fields, following the severance of her long connection with Harper & Row. Kahn's mystery imprint launched a thousand faces, but it must be geting harder for her to keep them aboard as the name on the contract keeps changing. However, St. M's assures fans that the latest Patrick McGinley (he made his debut with the highly praised Bogmail) will be on her fall '83 list . . . Diedrich Knickerbocker and Geoffrey Crayon, Gent. are one and the same: Washington Irving, whose bicentenary year is being celebrated in 1983. At Columbia University various Irvingiana is on display, including his unpublished work, The Chronicles of Ommiades. No, that's not a candidate for Judy-Lynn Del Rey's list at Ballantine; it's not even close to Brak the Barbarian. Rather, it's an epic history of the Mudejar monarchs of Iberia who ruled during the early Middle Ages.