The Final Reflection by John M. Ford
- The best star trek book there is. Totally at odds with any trek from TNG onwards since it was written in the eighties. Not a typical tos book as kirk & the enterprise are barely in it, and used solely as a framing device.
Federation(TOS + TNG), by Garfield and Judith Reeves-Stevens.
-It spans three eras and focuses on Zefram Cochrane(based on his TOS appearance, First Contact hadn't come out at this point). It feels like the kind of epic "The Motion Picture" tried and failed to be.
Memory Prime by Garfield and Judith Reeves-Stevens
Great take on artificial intelligence and conspiracies.
Yesterday's Son/Time for Yesterday by Ann C Crispin
A couple of great books that focus on spock and a rather surprising relationship.
Spock's World by Diane Duane
Deals with Vulcan -its past and what they think it's future should be. Includes an absolutely barnstorming McCoy speech.
Doctor's Orders by Diane Duane
Kirk puts McCoy in the captains chair, and things spiral out of control from there. Some great aliens to boot.
The "Errand" series by Kevin Ryan
-It's made up of two trilogies, Errand of Vengeance and Errand of Fury, which tell the story of the TOS era conflict between the federation and the klingons, split into basically four viewpoints - a starfleet command officer on earth, a security officer aboard the enterprise who is a klingon infiltrator, an officer aboard a klingon warship, and finally kirk.
One of the best flowing, and most satisfying trek sagas I've read.
Book order if you're interested:
The Edge of the Sword
River of Blood
Seeds of Rage
Demands of Honor
Sacrifices of War
Also, not strictly TOS, but close - Where Sea Meets Sky by Jerry Oltion, which is a very enjoyable cap'n pike exploration tale with a nautical feel. Was part of the "captains table" series, but doesn't require reading any of them.