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.