How to efficiently read mathematically and theoretically dense books in STEM fields?
I have never (as far as I can remember) read a technical book cover to cover from start to end.  Most of the time, I have a problem in mind that I want to solve, and I'm looking for tools to solve it; more often than not, if I'm trying to learn out of a book, I'm actually reading three or four books at once.  I dive into the middle of a book that seems most relevant; if I don't understand something, I'll back up, and if I don't understand that, I'll back up again, and if I get really stuck I'll put the first book down and pick up a more elementary book, and so on until I'm on firm ground again.  ("Getting really stuck" only happens after relying to work through/reconstruct details on my own, in addition to trying to understand them from the book's presentation.  I have taken months to read through one page, always feeling just close enough to understanding that I never felt "really stuck".)  Whenever possible I pop back up the reading stack with my target problem in mind, skipping entire chapters if they don't seem relevant (but backtracking if I discover later than I'm wrong), working forward again until I either find the tool that I'm looking for, conclude that I've been on a wild goose chase, or give up on the book.

Yes, I miss a lot this way.  Yes, I get a lot of weird ideas that I later have to kill off.  But I just don't have the patience to read large volumes of technical material that doesn't seem at least remotely relevant to some problem at hand, and prioritizing often leads me fairly quickly to tools that work.

Your mileage may vary.  Caveat lector.