"Death's Angel":
Oh come on, it's not that bad! There is something so unfettered about those Bantam novels - all they had to work on were the 3 seasons of the show, rather than everything that's accreted to them subsequently.
A lot depends on timing and mood. I'd just read several great ST adventures, bought second hand, and then "Death's Angel" turned up all shiny and new! With all its alien ambassadors, it should have been a favourite of mine - I'd just read "Journey to Babel" as a Blish adaptation, but not seen the episode itself.
I must have had my bullshit meter switched on for "Death's Angel" because every time an ambassador's race or name corresponded to an Earth term I got angry. For example, I
loved the little pyramid ambassador, who was mistaken for a table decoration. But his name was Hotep!!!!! The cetacean lady was... Sirenia. The black felinoid was Neko, which really does mean "cat". Etc. (I know, I know, TOS did that often, such as Lethe, Helen Noel and Mr Atoz, but these were early days for me.)
Then I met a new ST friend who'd just read the book and he thought it was the best comedy since "The Trouble With Tribbles". When I next met him, at a ST convention a few weeks later, he was dressed as Si-s-s-s (click) - and he won the costume parade. Years later, he was moving to England and he gave me the costume's blue crocodile head, which I still have. So, yeah, "Death's Angel" is one I kinda have a soft spot for, because Si-s-s-s (click) had some notoriety in Sydney fandom, including among fans who had never read the book.