Beau says:

For anyone who is trying to access their books today (Sunday, July 20) and not seeing their photos, the reason is some downtime with our provider.

To allow for effectively unlimited storage, your photos are stored securely on a system called Amazon S3 (yes, it's provided by the Amazon.com!). Currently, they are experiencing issues with their network and so your images are not available. We've been told that they should be fully online within a few hours, and not to be concerned about losing any, as it's a connectivity issue rather than anything else (e.g. your photos are still there, we just can't currently access them).

I will post an update here once we hear more.