If for some bizarre reason, I had to trim my library to just three books, you really can go so far with just Stick Control, Syncopation, and some imagination and some helpful interpretations provided by teachers and/or online suggestions.
For the third book, I'd have to go with something that the other two can't provide. This rules out something like New Breed, because as great as it can be, I feel like there are enough independence-based Syncopation interpretations out there that it doesn't deserve the coveted "third book" slot. Maybe something with advanced snare drum literature, like Delecluse's 12 Studies for Snare Drum or Cirone's Portraits in Rhythm. Or maybe something which provides a very in-depth look at a style-specific thing, such as Uribe's books on Afro-Cuban or Brazilian drumming, or Ed Thigpen's book on brushes, for example.