Mike
Having a fair few machining related books about the place I'd say that, in general, books are good for the how to's, set-ups and for gleaning some of the info your mentor would have passed on (assisted by obligatory head slaps) if you'd been lucky enough to get proper training in your youth.
I suggest that you first need to sort out what you need information on. Basically make a note of everything you have to go and look up and every time you find a job difficult.
Won't take long before you have a fair idea of what you need to look for. For specific info, such as feeds & speeds, get hold of data charts or copy them out of Machinerys' Handbook and make up your own folder. Use the transparent document wallet thingies in a loose leaf book to protect against grubby fingers. There is a lot about on the net, sometimes its the only easy resource for slightly off beat stuff so ramp up your searching skills, save useful sites and print off what you really need for your folder in case the site goes down. Three useful things I culled recently (and wished I'd had these past 20 years) were:-
1) a copy of American Machinist Cutting speed Chart (no 30) which is a nomogram sorting materials, diameter & rpm
2) a surface speed calculator graph scaled in Diameter & RPM with lines for various SFM speeds
3) a milling feed calculator graph scaled in teeth pre minute (i.e. RPM x no of cutter teeth) and feed in inches per minute with lines for the various tooth loads.
I also have a couple of cardboard slide & window type calculators giving milling cutter feeds and speed for various size cutters which I wouldn't be without as being the first port of call when I'm unsure after setting a job up on the mill. I'd miss the Cleveland Oracle data book and a similar publication from Presto for tap, thread, drill and some cutter info too. Like pretty much every UK based machinist I have a well foxed (pocket) and better (desk) copy of the Zeus data charts, first port of call for threading data, comparative drill sizes, bending allowances and aide memoir for sorting out what those pesky fit codes mean. Other stuff in there too.
I managed with How to Run a Lathe, Machinery's Handbook (late '40's vintage) and a Zeus for years until I could afford to buy books in quantity.
Clive
(who has done enough writing to know that producing a useful & informative book is a darn sight harder than any critic allows).