Inside Star Trek
is the closest thing to a definitive account, IMO. Loads of evidence in the form of memo's and interviews from a variety of those involved.
There was a nice coffee table book called Star Trek 365
, published a couple of years ago, which has lots of pictures and half page episode summaries - it was fun to read each entry after watching the episode.
When I was a kid my one and only Trek book was the Star Trek Compendium
- lots of facts and a short review of each episode, but god knows if you can get it these days.
A few notches down the accuracy scale, you could check out the spate of TOS actor autobiographies in the early 90's, but I'm not sure if they are still in print either...