I really enjoyed Vulcan's Glory (DC Fontana), The Wounded Sky, Spock's World (Diane Duane), The Lost Years, Demons, Mindshadow, Bloodthirst (JM Dillard), Sarek (AC Crispin), Diane Duane's Rihannsu series, Probe, Unspoken Truth (Margaret Wander Bonanno), The Pandora Principle (Carolyn Clowes), and Vulcan's Heart (Josepha Sherman and Susan Shwartz). Brad Ferguson is also enjoyable. I have yet to read any Peter David, but I hear he's good.
I personally didn't like Mind Meld, Sanctuary (John Vornholdt), The Joy Machine (Theodore Sturgeon and James Gunn), The Prometheus Effect (Sondra Marshek and Myrna Culbreath), Crucible: McCoy (David R George III - and I hear the Kirk and Spock aren't any better), Troublesome Minds (Dave Galanter), and The Better Man (Howard Weinstein). You might like some of these, but I thought they were either bad and/or boring.
If you're having trouble finding Trek books you like, find an author you like (for me, Diane Duane and JM Dillard) and read all of his/her books.