Some singers knock you down, bowl you over, and pride themselves on leaving you for dead; Shirley Horn blows bittersweet kisses that waft and float en route to intimate targets. Stretching languorously into a snail's pace ballad --such as "Lazy Afternoon" or "I Thought About You"--her style is unspectacular, almost artless, as she hovers somewhere between singing and talking. But her voice, soft, tiny, but indomitable, is like a slightly built karate expert: neither would be expected to pack the wallop that each of them does. The attention heaped on this deceptively fragile vocal style sometimes obscures the fact that she is a superb improvising pianist who does much more than merely accompany herself; indeed, I'd consider myself a little cheated by a Shirley Horn set without at least two strictly instrumental numbers. The next month or so shapes up as a stellar time for singers--with the far more extroverted Tania Maria and Carmen McRae due in the next few weeks--but I suspect the quiet, hazy presence of Shirley Horn will hover convincingly till spring. Tonight through Sunday, George's, 230 W. Kinzie; 644-2290.