The good folks at Desiring God pass along the following:
The sale has generated so much traffic to the checkout process in our store that the DG server is at its limit. In fact, the traffic is so high that even two servers couldn’t handle it. There are some solutions we can implement (namely, optimizing the code in the checkout process), but none that could be implemented by tomorrow.
Read the whole thing.
Basically, they are asking that people wait until tomorrow before trying to order books, given the unexpectedly high popularity of their unlimited, online-only $5/book sale.