Books

I got given a copy of this book for my birthday last year, The Ten Commandments of Typography/Type Heresy by Paul Felton. 

In the first section it details the 10 main rules of Typography. Flip the book over and read the second section and it tells you how and why you should break those rules.

It is also great looking and well designed without taking itself too seriously.

The other book I would recommend is Stop Stealing Sheep and Find Out How Type Works by Erik Spiekermann and E M Ginger. This is a great basic book on all aspects of Typography.