Star Trek Log Ten
was one of the first two ST books I owned (my mother picked it up for me in 1980 or 1981), so I've re-read it the most over the years, by far. Still one of the best early ST books, in my opinion. Alan Dean Foster adds quite a bit of material to his adaptation of "The Slaver Weapon" and it's still pretty good.
I've also read Star Trek Log Eight, The Entropy Effect, Black Fire, Web of the Romulans, The Wounded Sky,
and the novelizations of TWOK, TSFS, and TVH many times over the years.