iTunesU Courses: If you click these links on your iPad they will launch in the iTunesU app. Assuming you've already downloaded it.
iTeach Initiative - This was a course put together for preservice teachers, but when learning new tech stuff I often feel like one.
KN101: Using Keynote - If you plan on creating Keynote Presentations to embed in your iBook this might be a useful resource. You might be interested in some other courses created at Cedars School of Excellence as well. The have one on iBooks and another on Twitter.
Books:
Publishing with iBooks Author - O'Reilly
You can get this from O'Reilly's site directly and download it as a pdf file. I recommend this, it looks better.
You can also get it from the iBooks store as an epub document. The pdf looks better, but this one is easier to get..
Create Your FIrst Interactive Book - This is a really nice book created by the people behind iBook-Widgets.com
Miscellaneous:
Here are some pretty good resources. I grabbed this list from Anthony DiLaura's blog. Anthony teaches math on the west side of the state.
iBooks Author for Dummies - Mostly text and pictures. This is a good place to go after you've been playing for awhile. Not a great place to start.
Apps in Education Monster List - This includes many of the links I've already listed, but does provide some I do not.
Emerson College Tutorials - A little dry, but informative. These are all video tutorials
Apple's iBooks Author Overview - Official Apple tutorials
Tumult Hype - Hype is a program you can use to create animations for your iBooks. With a bit of learning you can make these interactive as well.
Extra Templates for iBooks - These are not free, but they're not to expensive. It might be worth getting one to set your work apart.