Mystery - Five mysteries. Selected from previously published stories, the five mysteries in Hansen's collection are ingenious but quite different from his novels about the gay private eye, Brandstetter. Here the chief character is retired sheriff Hank Bohannon, owner-manager of a California horse ranch. The old man leaves his haven only when called upon to search for evidence of criminal guilt or innocence, in the case of ''Snipe Hunt,'' for instance. A pretty girl, Billie Shears, begs Bohannon to save a youth she loves, Dennis Toomy, codefendant with Melanie Kruger, on the charge of murdering her husband, Thomas, a rich banker. The police have letters exchanged by Dennis and Melanie that virtually guarantee their conviction. Of all the stories, this one represents most impressively a purely deductive process and a spectacular upset. Conversely, the theme in ''Witch's Broom'' is relentless suspense, with Bohannon racing the clock to save a small kidnap victim. There is enough variety in this mixed bag to satisfy any mystery fan.