SINGAPORE by John Ball (Dodd, Mead: $14.95). Virgil Tibbs first appeared just over 20 years ago in John Ball's "In the Heat of the Night," the movie of which provided celebrity for the black homicide investigator from Pasadena. There have been five Tibbs novels since, the latest of which is "Singapore." Tibbs is still a gentleman and a courageous cop--in fact, he is shot helping a fellow officer at the scene of an armed robbery as this novel opens. The day he is released from the hospital, government agents descend. Would he, they wonder, help the state department out of a sticky situation? Would he go to Singapore to undo the murder frame built (by communists, of course) around Miriam Motamboru? (If you have kept up with Tibbs, you remember her from "Then Came Violence" (1980).) He would, and does. Interwoven with Tibbs' detecting on behalf of Madame Motamboru is a baffling child-murder case being handled by the Singapore police. On that, Tibbs offers advice with world-class tact--all his dealings with officials on this exotic island are exceptionally polite and dignified. Indeed, "Singapore's" appeal is not so much the detecting, which is a trifle obvious, as the fine manners, especially the impeccable conduct of officer Tibbs. Who else could share a bed with a beautiful woman yet never touch her?