There are many many great TOS books. Two recent ones that are now among my favorites:
Ex Machina by Christopher L Bennett
Forgotten History by Christopher L Bennett
Both are set just after TMP. Forgotten History is a prequel/sequel to Christopher's DTI: Watching the Clock, but it stands on its own well (But I highly recommend Watching the Clock. Its not TOS, but it is great).
"Canon is only important to certain people because they have to cling to their knowledge of the minutiae. Open your mind! Be a Star Trek fan and open your mind and say, 'Where does Star Trek want to take me now'." - Leonard Nimoy