I'm getting the same error with AGaramond Pro on another book today. The books both uploaded thru iTunes Producer to iBookstore without error though. I'm thinking it is the little js application called ePubCheck_3.0b5 (for Mac).
epubcheck-3.0b5-pre.jar - epub-samples - EPUB 3 Sample Documents - Google Project Hosting
Here is one chapter from the previous book though in epub 2. (that seemed to work better).
Umm. Hmm. I don't know how to attach it. Oh wait. Here's my public folder: