Wow, so many good recommendations here so far on where to expand my Trek reading, thanks to all.
As for the OP, if you're talking about the Trek book which has had the most impact on myself as a reader then it's the Destiny trilogy by Mack. I don't drink and I don't smoke but man, I felt like I needed both after finishing this beast. The most layered, deep, intense Trek book I've read so far, absolutely amazing in breadth and scope.
Now if you're talking the Trek book I've re-read the most, Federation by Reeves-Stevens. The crossover that should've been on film but for me, it's the story of Zefram Cochrane. Forget First Contact, how the authors weaved various bits of Trek lore & mythos into Zefram's back story, for me, this is my head canon of his true past. Moreover, his characterization felt more in line with the character as presented in 'Metamorphsis' than what I saw on the silver screen.
Honorable mentions to another Reeves-Stevens trilogy, DS9's Millenium, another amazing romp and DTI novels Watching the Clock and Forgotten History. I love me some time travel and these two novels do a bang up job of tying up/tying with time travel incidents throughout Trek history. Incredible work indeed.