There is a large number of literature about physics engines available online. The problem is, some tutorials and papers are based on outdated methods (constraint forces for resting and colliding contacts); others are rather scientific and hard to deal with if one doesn't have an intuitive understanding of the material beforehand; and yet others simply omit the hard parts or the math and show the "obvious" parts of game physics, only. Here is an ordered list of material I found very useful:
Tonge (2013) - very good overview over PhysX. Read this to get started with physics engines. I'd skip the LCP part if you are new to it.
Catto (2009) - Excellent introduction to impulse-based constraint solvers. Also contains slimmed down version of Box2D.
Slides and paper of Erwin Coumans' Bullet 3.0 rewrite to include GPU support. Contains a lot of useful information about Bullet internals.
Once you know the jargon - velocity vs. force constraints, LCPs, AABBs etc. - it's easy to find more in-depth material.