Great thing about physical handbooks, whatever the size, is that you get to know where the stuff you need to look up or always forget is.
Internet tends to be more variable. Sometimes favourite "go to" sites change or even disappear. RoyMech(?) was great.
I have 3 hard copies of Machinery's. All got second hand to be affordable. Last one is the large desktop version. Not a lot of use but when I need them I need them. Same with Caxtons, Newness, Kempes et all. All have something the others don't.
Older ones are more important to me than most as I see a lot of old stuff that needs fixing.
That said Zeus books are the ones I use most. 3 in the workshop and one in the house for instant information.
I have several other compact sources too.
I wonder if a comparison article covering the modern, smaller, references would be useful to MEW readers showing what you have in which one. Agree that Zeus is probably the one to get first as its small inexpensive and contains much of the need it in your pocket data.
But which to get next could be a puzzle.
I think I've picked up the lot but I'm an info junkie and there are significant differences making the best one dependant on what you do. Even Zeus editions vary.
Clive