Speaking as a certified GCIA, the exams will be an order of magnitude easier with the courseware ("official study material") than with related books. I've also certified GCFA, a couple other expired certs, and worked with the exam writing teams some number of years ago. They are constrained to make sure that exam questions are directly supported by the courseware (for the obvious reason), so there's a distinct advantage to using courseware - if it's in the exam, you know it has to be in the courseware somewhere.
They used to sell it for something like $500; I understand they've stopped selling it online. If you can physically get to a conference, or make an arrangement with someone, they do sell the courseware in the bookstore during the conference. I don't think you need a conference pass to get into the conference bookstore, but I could be wrong.
You are correct that GIAC offers a Challenge exam. You get practice exams with it, so if you wanted to prep with other books, that would give you an indication as to whether they were enough before you actually take the pass/fail test.
My advice: if you're looking at buying a $200 pile of books versus the $500 courseware, I'd consider the additional $300 well spent, given that the Challenge exam is $900 on the line.