Have you tried sending an e-mail to Wess or Bagger? In my experience, even very busy professors are quite receptive to answering fan mail related to their textbooks as they get rather little of it relative to other kinds of authors and feel some special sense of obligation as they frequently do multiple editions of their books eventually and get a royalty in addition to their usual paychecks for it. The e-mails are usually easily available on faculty web pages.
While professors tend to dislike questions from their low level undergraduate classes, they tend to enjoy serious questions from advanced students even if they are technical and narrow.