If we're including Vanguard which only has occasional Kirk Spock and Co. visits (two I think), I'm including The Eugenics Wars...
Final frontier & Best Destiny by Diane Carey are good novels. The Myriad Universe story Chimes at Midnigth an au look at the Startrek 3 movie is really one of my favorite stories from the myriad universe books. Constellations Startrek Tos short story anthology and Mere Anarchy are excellent stories by different Startrek writers. Crossroad by Barbara Hambly. The wrath of Khan and Search for Spock novels by Vonda McIntire.
Just to be clear, I was recommending both books. The Yesterday Saga encompasses both Yesterday's Son and Time for Yesterday.
I'm going to second My Enemy, My Ally and Fedeeation. Also really enjoyed Crossover and Sarek. Although I was a teen when I read those.
Cast No Shadow is not a TOS book. It has Spock in it, but it takes place in 2300. It fits more as a Lost Era novel.