A bunch of people on the flist have been making or asking for recommendations for classic Trek novels, so I started digging in my library to see what I had thought was worth keeping through multiple moves over multiple years. It's been a while since I've read any of them, so YMMV (and at least one case, I'm looking at it going "You must be joking, I kept Spock Messiah? What in hell FOR?")
But these, off the top of my head, are the books worth having, if you can find 'em. Some are obscure, all are long out of print. Get 'em on ebay before the demand drives up the price.
Vulcan Academy Murders by Jean Lorrah. Frankly, her fanfic was better because she wasn't stuck within Paramount's limits. (If you can score any of the Twin Moons fanzines,
grab 'em quick!) But she had an understanding of Sarek and Amanda as a loving couple that is unparalleled. Murders centers very much on them as opposed to the TOS Enterprise crew, although there is, naturally, plenty of Spock and some of Kirk & McCoy.
How Much For Just the Planet by John Ford. The crackfic against which all crackfic is measured. How Paramount let the man write a musical comedy as a tie-in novel may never be known, but imagine The Trouble With Tribbles with musical interludes and the insanity cranked up to 11. [ETA: My memorial post
A Book With Yellow Pages was about this novel.]
Dreadnaught! and
Battlestations by Diane Carey. Yeah, Piper's a bit of a Mary Sue, but so's Mary Russell. If you like Mary, you'll probably like Piper, especially as Piper does not marry a main character.
Ishmael by Barbara Hambly. This one will be tough to find because it disappeared off the shelves as soon as Paramount belatedly realized that Hambly had managed to write an unlicensed crossover with Here Come the Brides without them noticing. (I may have kept this more for the novelty of owning it rather than the merits of the novel.)
The Romulan Way by Diane Duane and Peter Morwood. I list more Duanes below, but this one was a particular favorite, focusing on a federation sleeper agent among the Romulans. The Romulans are treated with dignity and respect, which is one reason I liked it; another is that the only TOS character to have a large role is McCoy.
Spock's World by Diane Duane. A long-term history of Vulcan and the evolution of its people. She has some bits with their language that linguists will appreciate.
Uhura's Song by Janet Kagan. Everyone in TOS is in it, but as the title suggests, Uhura carries the story. Lots of vibrant worldbuilding as they seek for the cure to a plague on a planet of intelligent felines.
These books I haven't read in so long that I'm not sure if I should be recommending them or not, because I don't really remember a thing about them. However, I found 'em worth keeping at the time, so I'll list them:
Pretty much everything else by Diane Duane. I've got
Doctor's Orders,
The Wounded Sky, and
My Enemy, My Ally.
Crisis on Centaurus by Brad Ferguson. After listing what action the K/S/M trilogy is up to, the blurb on the back continues "Now Lt. Uhura, left alone in command, must jeopardize the crippled Enterprise to save, Centaurus, Kirk - and Joanna McCoy!" So probably one for the Uhura fans.
Demons by J.M. Dillard
There was also something about a world of singing fur seals that I remember enjoying and which also had a lot of Uhura in it, but it's not coming to hand tonight and I don't remember the name. {ETA: Named in comments.]