Dark Mirror -- Easily my favorite Star Trek book ever.
Crossroad -- probably my favorite TOS novel
Home is the Hunter -- brilliant and overlooked character studies on Chekhov, Sulu and Scott
Prime Directive -- read at least 4 times, never disappoints
Probe -- read this once about 20 years ago and it stuck with me. I need to re-read this one to see if it's as good as I remember.