Q-Squared.
I re-read it so many times over a few months that my library wouldn't let me re-borrow it any more. Then when I bought a copy, I re-read that many more times over the years.
I got quite a bit out of all those re-reads, since I wasn't too familiar with Star Trek when I first read it. My increasing knowledge helped me pick up on more of the episode references each time.
It's still my favourite Trek book.
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Kim: I'm detecting some weird technobabble.
Janeway: A possibly dangerous anomaly that we know nothing about?
Kim: Yeah. I suppose we should steer clear.
Janeway: Ha! Good one, Ensign. Take us in, Mr. Paris.
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