These sites provide reference material or background information. Also listed here are two books on building CNC routers that I consulted along the way.

User's pages on CNC & Machining
John C Kleinbauer's Unique Plans for Hobbyists - see section on workshop & tools - cnc conversion info
Dave Hylands - machinist stuff
Tauseef's - CNC Machining and RC Helicopter Webpages (
Build Your Own CNC - from MDF - video step by step
brusselsprout - cambam, pcb isolation routing, diy cnc machine
How-To: Build your own CNC machine (Part 1) - engadget

Machinist's  References
USC iLab CNC wiki - speeds and feeds, links & info on tools           
speed&feed calculator -               
mill&drill calculators  -
drill tap & clearance hole chart  -  
conversion tables -

G-Code References
G-Code Quick Reference - linux emc2
G-Code Programming Basics - linux emc2
G-Code Reference - for mills and lathes
CNC Forum for G-Code questions
CNCezPRO - G code reference page

Books - don't buy either one. See if you can borrow from your local library.
Build your own CNC machine by Patrick Hood-Daniel, James Floyd Kelly.
2009 - Apress. - ISBN: 9781430224891
CNC robotics : build your own workshop bot by Geoff Williams.
2003 - McGraw-Hill. - ISBN: 0071418288