The American Society for Precision Engineers 2007 meeting just came to a close, and Alexander Slocum (who wrote Precision Machine Design, a standard industry reference) has a new "book" he put out for free. It deals with the principles of design practice, and I suggest everyone here who wants to build a CNC read through it.

http://pergatory.mit.edu/2.007/fundamentals/

It is written for folks who don't necessarily have a background in machine design (like Precision Machine Design is), but want to develop the good skills necessary to be more successful in their designs.