books
I would start with the book that came with my machine.
If it is a home built machine then start with the machine control software book. It's a good idea to understand what the machine wants to start.
G and M codes are easy if you understand where they go. The machine control book will give good examples of what to place where.
I would not spend any money until I understand what my machine wants.
After this you willl be suprised, books are not as machine specific just full of general info.
You might end up buying a book but between this site, and the machine control book you should be in Great shape.
A G and M code list is here on the site, but they will be different for a lot of machines.
I hope this helps, and saves some $..
Later...
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Thanks Marc