The most important books in an engineering workshop are data tables and counsellors (they are probably the same thing come to think of it!)
If you are going to buy only one data book make sure it is the ‘Zeus’ data chart book. I still use my Zeus book I bought as an apprentice in 1974, it cost 50p! and a few years later I bought an updated Zeus (metric revision) for £1.60! that’s inflation for you!
Inside the books is all sorts of data, including drill sizes, conversion charts, tapping sizes including all you need to know about thread forms, how to sharpen a drill, how to find the angle of tapers, limits and fits, co-ordinates for drilling holes and much much more.
You can get these books from good book stores or better still from the internet book stores like Amazon. Another source is trade stalls at engineering fairs or model engineering events, where sometimes you may pick them up for free, some manufacturers produce their own, such as Presto, Sandvick, Starrett, Mitutoyo etc. And of course all this information is available from the internet, but nothing is handier than having a dog eared copy of the Zeus book in your top pocket or drawer.