First round: My first ST novel, and the book that made me realize that there were ST novels out there, was a 1987 Christmas present:
Encounter at Farpoint. When the two-hour episode was originally on the air, I got to watch it twice (7pm & 11pm, same day, same channel). When they replayed it in December as two, separate, one-hour shows, I got to watch the first half, twice again; but when the second half aired, campus was closed, and my parents didn't receive any channel that had it on the air.
So, after I started to read the novelization, I realized,
Hey! This is the first episode in book form! So the first half of the book was very familiar--I could still picture many of the scenes and lines, because I had recently rewatched the first hour. However, during the second half I was totally lost. It had been too long since it had first broadcast for me to remember it.
Because of this intro to Trek lit, I thought that all ST novels were based on episodes. Because I had seen almost all of the TNG episodes when they originally aired, but wasn't sure if I had seen all of the TOS episodes yet, I only bought TNG novels back then. Yet, I could never recognize the episodes they were based on. I figured that that was probably because they weren't fresh on my mind. It wasn't until many years later that I realized that
Encounter at Farpoint was the exception and not the rule.
Second (current) round: My first novel after getting back into ST and ST lit was
Spock's World, which I hope to reread again some day soon. Around the time I read it the first time, I had read something online (possibly at TrekBBS, possibly not) that Diane Duane had been told to not make it consistent with previous novels. In part because of this, I thought, as I read it, that she was making all sorts of things up about Vulcans, such as how they are really highly emotional, and have to learn how to control their emotions. Having watched a lot of other sources, I realize now that much of those "illogical" statements were really canon "facts". Also, some of the charcters in the novel I'm now more familiar with, such as Sarek and Amanda, for instance. By rereading it, I hope to gain a new appreciation for it that was missing the first time through.