Introduction: Basic Machining Information/textbooks
The US military have a number of excellent manuals for the use of their machinists -- and the text is public domain. Your tax dollars at work!
I've prepared some single-file PDF's which are a bit easier to deal with than the more usual single-chapter-per-file setup.
If you know of sources for other useful public-domain manuals, let me know and I will add them here.
Please link to this page so people can see what else is available (i.e. don't deep-link).
US Army Fundamentals of Machine Tools.pdf is 7Mb, 300 pages, 1996
US Navy Machinery Repairman Handbook.pdf is 15Mb, 430 pages, 1993
US Army Theory of Welding and Application.pdf is 20MB, 720 pages