While even just reading Pocket's complete Star Trek output is a monumental undertaking, I find it a bit disappointing that you didn't start with the Bantam novels and the Ballantine Star Trek Logs -- or even Mission to Horatius if you really wanted to be complete about it, although I've never read that one myself.
Although since you've already started reading in chronological order, it's a bit too late for that -- although you could still slot in Bantam's Perry's Planet, The Galactic Whirlpool, and Death's Angel, which came out between the TMP novelization and The Entropy Effect (which is why TEE was delayed so long -- Pocket obtained the license in '79 but had to wait until Bantam had released all the books they had under contract before that point). Although the only one of those I'd actually recommend is Whirlpool.
I'm not actually reading in chronological order, so I could go back and include them. However, at the moment, the reading order is publishing order, starting with the ones I already have and gradually going back and fitting in the ones I don't. I have brought a couple in order to read the first 4 at least, but then I'll be skipping a few which I'll come back to later. So I may branch off to some of the Bantam ones later on.