3 ,17, 37,38 and 42 are the ones i found most useful but the entire series is a "must have " as far as I'm concerned .
The data book is pretty useful and i found it was the one i reached for first as it is easy to use and not full of information that you will more than likely never use , i feel machinerys hand book is a bit like that !
Sooner or later you will need to :
Sharpen your cutting tools
Use gears
Cut threads
Or use a dividing device of some sort to either cut gears or make a dial etc .
I have read the lot ,I can't say there is one bad book amongst them and if some bright spark could put them into one big book it would be fantastic !