I have told this story before, but Gene Roddenberry's "Star Trek: The Motion Picture" novelization made me a lifetime Trek fan.
Topic of conversation at my 21st birthday party in December 1979 was my school friend's enthusiastic review of the Sydney gala premiere of TMP. I had to wait another week for the movie to start its theatrical run, by which time I had read a Aussie journalist's serialized visit to the set in the afternoon newspaper, used a birthday gift voucher to buy the soundtrack LP, and found the book on a spinner rack at my local supermarket. I started reading the novelization while awaiting someone to agree to go to the movie with me. I had planned to stop reading about halfway through, to maintain surprise at the theatre, but couldn't put it down. I ended up going to the movie alone. At least five times over the next few weeks.
Until that book, my only exposure to Trek had been about eight episodes of TOS when colour TV came Down Under (March 1975) and random episodes of TAS (in b/w first-run on Saturday mornings, and then repeats in colour in 1975).
I owe my fandom experience to a Trek book.
Had several crossover episodes with "Phase II', starring David Gautreaux as Xon.