Should I send an email to the author of a book/paper asking for clarification?
If a work (book or otherwise) is assuming you know a concept, yet you can't find information about them elsewhere, then it's likely that either you're missing something fundamental or else the concept may itself be problematic.  I had an experience of the latter sort once reading a book about "holons" that all sounded very nice if you were just sort of reading causally but ultimately turned out to be basically pointless handwaving.

In either case, it's always OK to write to the author of a scientific work to ask about their work.  Don't expect them to teach you a course, however.  Instead, you can say something like:


  I'm reading your book "Weasel-Taming in but a Single Fortnite", and although I find it very interesting, I'm not familiar with some of the things it assumes as background. Would you be willing to give me some pointers on where to learn about the Voight-Kampff test or how to temper chocolate?


substituting the appropriate nouns, of course.  If your email doesn't get lost in their inbox, most authors will be more than happy to at least give you a hint for where to go learn about their favorite subjects.