I definitely think GEB is important to getting into IAASL, which is why after I got into the book a bit I put it down and really made an effort to get into GEB. IAASL can totally be read on it's own, but I think it's fair to say you at least want to have a copy of GEB around to look at while you're reading it.

I can't recommend that course enough too. I guess I couldn't find the book on my own. So anyone that can, great! But having someone who knows the book take your through it, at least for me, made it a lot more fun. It was, in my experience, kind of a rare book in how I wanted to share what I was looking at with someone but didn't have anyone to share it with really. Having the prof giving a breakdown made it a lot more digestable for me.