Reddit reviews Machine Shop Practice, Vol. 1

I recommend Machine Shop Practice Vol. 1 & 2 to get a sense of the machinist's trade. You will have no idea what half the stuff in the book means until you've spent a few months on a conventional lathe or mill, but the reference will be extremely helpful as you progress. There's next to nothing about CNC work in there, unfortunately, but it's one of the best general references I know for this work.