So far I've only read 3 TOS novels, Troublesome Minds, Strangers from the Sky, and Spock's World, and I really enjoyed all three of them.
There also some semi-TOS books that are worth checking out:
The Vanguard series: A 7 ( and two e-book novella) series following a TOS era space station in an unexplored and mysterious region of space. It's a fascinating, morally complex story dealing with galactic history, and interplanetary politics. Seriously amazing stuff.
The Lost Era: The Sundered: An Excelsior story dealing with the Tholians and a new alien race.
Excelsior: Forged in Fire: Another Excelsior story by the guys who wrote The Sundered. This time they're dealing with the beginning of Sulu captaincy of the Excelsior, and the origins of the Klingon Captains' conflict with The Albino in the DS9 episode "Blood Oath".
The Lost Era: Serpents Among the Ruins: This one deals with the Enerprise-B and tells the story of The Tomed Incident referred to in TNG.
The Lost Era: The Art of The Impossible: Follows numerous characters through the 16 year Betreka Nebula Incident involving the Klingons and Cardassians. Featured characters include Curzon Dax, Mogh, Elias Vaughn, Sarek, Enabrin Tain, and many more familiar faces.