I think the first ST book I read was almost the novelization of The Motion Picture. I was starting to read it, and I was very, very young. Naturally my eyes bugged out when Kirk uses stronger language than I had ever encountered before. There was this mystery to me, about a ST movie that I didn't even know about...Wrath of Khan was well liked and my family had seen it, and I think I went to the theater to see the Search For Spock, but information about an earlier ST movie was hard to come by. So reading the novelization was exciting, because otherwise I had no way of uncovering the mystery of the unknowable first ST movie. And then I lost the book, only a couple of chapters in. It just vanished, And The Motion Picture continued to be elusive for several more years. Eventually I was able to watch it, and eventually get a new copy of the book to read.
The one that ended up being my first read was Diane Carey's Final Frontier. It was a long reading commitment, being still a very young reader at the time. It was exciting learning about the history of the ship and crew that no one seemed to know. A fun book that I still have to this day, and periodically will read through every couple of years.