I've always found a good way to pick text books and other reading material for a specific course is to find the university with the leading department for that subject, then find the syllabus for a class in that course taught there. My experience has been that it's rare to find a top university where you can't find syllabi (if not full course sites with powerpoints and assignments) online with some Google sleuthing.
Back in college, there were a few courses where my own professor's lectures were difficult to follow. I almost always was able to find additional material and often full PowerPoint decks from equivalent courses at other universities to help out.
"I've always found a good way to pick text books and other reading material for a specific course is to find the university with the leading department for that subject, then find the syllabus for a class in that course taught there."
This is a great way to find good texts. A wrinkle I add is to check the prerequisites for a course and repeat the process for each prerequisite till you have an ordered list of texts. Working through a text is much easier if you have nailed the pre requisites.
I used to do this too, ordering a big box of books from syllabus of prestigious universities at a level that was often way beyond me.
I usually flipped through them, and picked up a few things, but in the end I realized I had to really understand linear algebra, or number theory, or whatever, to really grok what was going on.
Now I knew I needed these prerequisites and why, not that someone just told me so. This motivated me a lot more to get through (or nail) the prerequisites.
And, depending on the field, one good way to get inexpensive reading material is to pick Latest.Edition - 1. For many courses, the incremental changes are small and the field doesn't advance fast enough to make the new book worth way more than the old book.
Back in college, there were a few courses where my own professor's lectures were difficult to follow. I almost always was able to find additional material and often full PowerPoint decks from equivalent courses at other universities to help out.