Christer Ericson and Gino van den Bergen both have solid and pretty accessible books on collision detection. Christer's Real Time Collision Detection, in particular, is really digestible in bits and pieces as needed for various applications (and he provides code for each example which rocks).

Real Time Collision Detection is great, but leaves almost all capsule collisions to intuition. That was fine for me, but probably pretty unsuitable for people who aren't very good at geometric visualization.