For non-fiction I'd usually go to the bookstore and pick the thinnest book on the subject. I don't like filler in nonfiction books. My favorite thing about them is if I don't understand a difficult passage I can go back and read it again until I do. Among others, this technique has gotten me Relativity[1] and QED[2] to name just a couple.
Oh wow I never would have thought of doing it this way. I definitely choose books for their covers at times, but this makes way more logical sense. Although I do feel a good cover is a decent signal because it’s nontrivial to get that right, and if they were able to do that then they probably got other things right in the book too.
[1] https://www.goodreads.com/book/show/15852.Relativity
[2] https://www.goodreads.com/book/show/5552.QED