Read many CNC and Machining books from Google books and others.
Though some are not complete, they still provide valuable information for free.
Select a Book
CNC Programming Handbook 2nd Edition by Peter Smid
CNC Machining Handbook By James Madison
Theory and Design of CNC Systems
CNC Control Setup for Milling and Turning
Fundamentals of Machining and Machine Tools
Machinists' and Metalworkers' Pocket Reference
American Machinists' Handbook and Dictionary
Lathe Work for Beginners
Modern Machine Shop Practice Vol 1 PDF
Modern Machine Shop Practice Vol 2 PDF
Marks Free Technical Ebooks links - must see!
ASM Handbook - Aluminum and Aluminum Alloys