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