Engadget highlights the economics of publishing iBooks Textbooks to the iBookstore, noting that paid downloads follow Apple’s traditional App Store and iBookstore model where the company takes a 30% cut of the purchase price of each book. Authors are free to set their own pricing, although Apple has placed a maximum price tag of $14.99 on the textbooks.

In addition, the report notes that Apple requires all iBooks Textbooks to be exclusive to the iBookstore. An iBooks Author app for Mac to create their iBooks Textbooks for distribution through the iBookstore.
Update: There seem to be some questions about the sourcing and completeness of Engadget‘s claims. Apple states in its iBooks Author license that all works created through the software to be offered for sale can only be sold through Apple. But it appears that authors may be able to use other tools to offer those same titles on other platforms on marketplaces as long as they do not use iBooks Author to create the files. As TechCrunch notes, the output of iBooks Author is based on ePub but not technically in the ePub format, so those files likely would not be compatible with platforms other than iBooks anyway.
Books are not technically in the EPUB format, but they borrow from it (likely EPUB 3). Certain interactive elements of the books require the files to be done in the slightly different iBooks format, Apple says.
In light of the uncertainty about the situation, we have moved this post to the iOS Blog.