The CNC Cookbook: An Introduction to the Creation and Operation of Computer Controlled Mills, Router Tables, Lathes, and More
This is a really good introduction to CNC. It's well written and understandable. It will help you understand some of the vocabulary and gives a good introduction to CNC.
Wikibook (nascent) Hobbyist CNC Machining
CNC router evaluation procedures by Edward L. Adams
G codes for the specification of Pythagorean-hodograph tool paths --- Academic paper discussing how curves, arcs and circles are handled and the consequences of representing them as polygons or polylines.