I like all of Vornholt's Trek novels. I feel he describes the alien worlds and cultures so well. War Drums is very much a favorite. Someone also mentioned The Lost Years (or Lost Era). I like those as well. Covenant of the Crown also.I remember liking Deep Domain quite a bit, also.
War Drums by John Vornholt was one of the adventure of the week TNG novels that came out during the run of the series. I don't remember exactly why I enjoyed it so much, but do remember it featured Worf and Ensign Ro. It also had some resemblance to the "Birthright" two-parter, which must have been coincidental since that had not aired yet when the novel came out.
Another one that might be underappreciated, but I like very much is Ishmael. I mentioned it to someone on another Trek message board, they thought it sounded quite strange. Because of it being a crossover with another TV series.
Edited to add. After reading all of the comments in this thread, I see someone mentioned Ishmael.
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