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 The problem is, some tutorials and papers are based on outdated methods (constraint forces for resting and colliding contacts); others are rather scientific and hard to deal with if one doesn't have an intuitive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:

Once you know the jargon - velocity vs. force constraints, LCPs, AABBs etc. - it's easy to find more in-depth material.