Deborah Henson-Conant, Friday, June 23, at Swallow Hill Music Hall, is a jazz virtuoso and superior improviser with an unlikely specialty--the 47-string grand concert harp. A native Californian now based in New England, she first learned about the harp's limitless possibilities when she was hired to play a three-hour restaurant gig for which she had only thirty minutes of composed music. The discoveries that followed have led to five recordings filled with original pieces and cover versions of jazz, blues, boogie-woogie, Latin samba and flamenco numbers. On Naked Music, a 1994 release, Henson-Conant also began adding rich, honeyed vocals to the mix; her singing provides a stark contrast to her instrument's tones. The unconventional nature of the sounds that result is matched by Henson-Conant's stage attire, which often includes leather skirts, spiked heels and dresses marked by decolletage guaranteed to attract concertgoers' attention. But it's her music that's earned her a reputation for innovation. This woman really knows how to pluck.