I finally got Christer Ericson's "Real Time Collision Detection" which is a great comprehensive start. Bullet uses many of the algorithms (or variations thereof) that are described in the book, but no the book isn't specific to Bullet.
+1 on 'Realtime Collision Detection' recomendation.
I found Eberly's Game Physics a very helpful (and easier) supplement, especially for the broadphase 'sweep and prune' description... Good chapter on LCP's as well.
I don't use a ton of Eberly's methods, but found them very useful for introduction to the subject...