In particular, I'd recommend finding the first/foundational work (usually cited in the textbook itself), and reading that. One reason that foundational works are foundational is often that they manage to effectively communicate a concept to a large population. That is, not are they theoretically novel and groundbreaking but they are rhetorically competent. Re-tellings by others are often inferior.
(This isn't always the case, but it is often enough that it's a useful tip to keep in mind.)
Otherwise, look to see what other references are included in the text, and try reading the relevant sections of these. Today those are often available via the Internet Archive for check-out, or you can simply download books from Library Genesis / ZLibrary, or articles from those sources or Sci-Hub.
I also found this helpful: "The single most helpful thing in figuring out what to write down was noticing when my reading was slowing down, which typically meant either there was a particular fact that needed to be moved from short to long term storage, or that I needed to think about something." Here is the source: https://acesounderglass.com/2020/06/10/what-to-write-down-wh...