I have 2 Machineries handbooks,one kept clean indoors and the other stays in the workshop,mainly used for gear and gear cutting info,lots of info when needed though very US based,one was well used and a gift the other was works issue. The machineries screwthread book is used far more often, purchased it new in 1967 and is so useful when identifying threads on vintage work. I have Kempes engineers handbook (2 vols) not much use for our sort of work,can't grumble thy were again works issue.I was given some time ago a book published in 1905 "A text book of Mechanical engineering " written as a text book for mechanical engineers taking the city and guilds examination,covers loads of now historical information and prints of machine tools,hand tools,even a diagram how to cast the iron spokes into the centre hub of a traction engine ,just a good read.It sounds a bit silly but the most used booklets in my workshop are by Presto,Dormer and Moore and Wright,for drill sizes ,cutting speeds,tapers ,metric/ imperial conversion tables,etc etc and were free.plus the zeus type tables, though I do find the tapping diameters in my tables bought in 1958 at the start of my apprenticeship are a lot tighter than those specified in the more modern Zeus tables. One set of books which so good are the 4 volumes of treatise on the steam engine by DK clarke,two volumes cover the description in minute detail of smaller stationary engines,portables,up to the larges mill and marine engines,and railway engines but no road steam.The drawings are so fine and clear,show all the fine detail,they would be ideal for someone making a serious model,the other 2 volumes cover boiler work and steam theory and not so interesting,they were a bargain at £25 from a neighbour.