As far as official, sanctioned works go, the contemporaneous (and thus out-of-print) The World of Star Trek and The Making of Star Trek, along with Chekov’s Enterprise and The Making of Star Trek: The Motion Picture all offer a good (if probably thoroughly scrubbed) look at how the original series and the first movie were made. I’ve not gotten a chance to read Allan Asherman’s The Making of Star Trek II, but from what I understand, it’s illuminating and goes into more depth than the other books about how the cinematic sausage is formed.
More recently, Solow and Justman’s Inside Star Trek: The Real Story (which is also out of print but easy enough to find) offers a true inside look at the show’s production and features a number of anecdotes that the Roddenberry family wishes weren’t in the public eye. I’ve been reading the recently-released (and massive) Creature Features oral history of Star Trek: The Motion Picture in bits and bites and while I think the narrative could be constructed a bit better — stories about writing, casting and special effects get tossed together pretty willy-nilly — there’s still an awful lot of great stuff in there.
The best bang for the buck, though? The These Are The Voyages books. They’re meticulously researched and so well done that I feel like I’m giving away my secret for this Tumblr by recommending them at all. If I’m stuck on coming up with an anecdote for any given episode, I just open up the book, go the relevant chapter and there’s something worth writing about. Terrific, terrific work by Cushman and Osborn.