I think they goofed. I got book 2 two weeks ago, and it was definitely marked at price $0.00. Put it in my "shopping basket" and checked out. I didn't care for book 3 so didn't try last week. Wanted book 4, and I agree with you guys -- they've forgotten to put it down to $0. Who is brave enough to remind them? Or knows what address to email to?
OK -- I wrote them a polite note. Let us know here if you notice the price get changed!