If you can find a copy, I'd highly recommend "The Entropy Effect"
by Vonda N. McIntyre. It's my favorite TOS
novel for sure, and probably my favorite Trek
novel in general.
Coincidentally, I just
finished reading it again on my Kindle Paperwhite this morning. I have quite a few more Trek
ebooks I'm looking forward to reading on it.
Another good TOS
novel is "The Kobayashi Maru" by Julia Ecklar. I also recommend "Spock Must Die" by James Blish.