I used the Safari Books Online "Safari to Go" app to download a book to its "Offline Bookbag". In what format is the book stored? I ask because I'm hoping to use a reader other than Safari's own app to read the downloaded book.
On my Nexus 7, my downloaded books go in
While these look vaguely HTML like, they certainly can't be rendered in Chrome or other web browsers directly, you just end up seeing all of the mark-up as text.
I don't know how standard the xhtml format is for e-books, or whether other readers could understand the format Safari uses, but hopefully this helps point you in the right direction.