I really liked Yesterday's son and Time for yesterday. novels books they're some of my all time favorite Tos Books. You also might like Sarek too by A.C. Crispin. No spoilers but I thought the stories were excellent. I've re-read several times. Greg Cox wrote a TOS / Voyager crossover Novel a few years ago that kind of ties in with these books too. Somewhere in his book he said A.C. Crispin books in spired this story.
Slowly working my way through the first ST books now in order. Just expanded my library to just under 300 books - half of which I have read. Plenty to keep me busy.
Spock's World was like three books in one the war like time ; the birth of logic ; and the logic time with questionable thinking
Great stories, please read in order you will appreciate it more. Spock's World and Sarek are also worth reading also the story Vulcan forge/heart/soul series. I find the OC more interesting than the canon ones.