Spock must die! was a 1970 Star Trek novel by James Blish. I read it as a kid and reread it recently after coming across a used copy. It was an enjoyable read as long as I stayed determined not to let myself focus on some of the silly and nonsensical stuff that cropped up here and there.
The novel introduces some interesting material that I didn't recall ever seeing in the TV and movie incarnations of the franchise. Some of this seems clearly disconfirmed by more canonical stories, but what about the following?
McCoy is given a backstory in which he joins starfleet after a divorce.
Chapel has some sort of similar vaguely defined romantic backstory.
Multiple human women on the ship, not just Chapel, are said to desire Spock romantically or sexually.
Some other stuff seems totally inconsistent with canon, although I could be wrong:
Kirk is described as having been a starship officer and captain for longer than seems possible given his mid-30's age as portrayed in the TV ToS.
The Klingons are locked up in some sort of metaphysical jail for 1000 years.
Rand is portrayed as more of a Maturin to Kirk's Aubrey, rather than as a nameless ensign whom he barely considers worthy of attention.
Starships under warp drive are described as having their geometrical shapes weirdly distorted as seen by eye.
There is some description of how in space, it's incredibly hard to engage the enemy in a naval battle. (This seems scientifically sensible but obviously not much like the TV and movie depictions.)