In The Best of Taxicab Confessions, a burned-out rescue-squad cop describes the horrors he’s seen. A young slacker cruising for hookers reveals his tragic family history. A woman on the verge of meltdown ominously recalls the time she shot her cheating boyfriend. Culled from the Emmy-winning HBO series, these are among the New York cab riders whose frequently startling conversations have been captured by Joe and Harry Gantz with hidden cameras. While watching these unwitting subjects spill their guts, voyeur’s guilt gives way to sheer fascination — and wonder at why they granted permission to the sneaky filmmakers.