Bookshare Books: If your school or your student is a member, the easiest way to read Bookshare books on an iPad is through the Read 2 Go app ($19.99). You can also download the Bookshare book on a computer, convert to a Text file or Word file, and save as a PDF. Once the PDF of the book is created it can be opened in the free iBooks app. The iPad Voiceover accessibility feature can be enabled, and then the iBook PDF will read aloud. (Somewhat cumbersome to get this done, but it is possible).
Library Books: The free Overdrive app allows you to download ebooks and audiobooks from the public library. Audiobook titles are limited, but some of the classics are there (and these are commercially produced, human read books, not synthetic voice).
Commercially Produced: There is a free app for books from Audible.com. (You have to pay for audiobooks – $14.95 per month which earns you credits toward books, or you can pay additionally for more books). Audiobooks, by Simply Audiobooks, Inc. – $29.95 per month for unlimited streaming of over 11,000 audio books.
Read this article from PadGadget reviewing some of the options.