O'Reilly has two books available free on the topic of epub3:
What Is Epub3
http://shop.oreilly.com/product/0636920022442.do
Accessible Epub3
http://shop.oreilly.com/product/0636920025283.do
"What Is Epub3?" is only 24 pages of reading. The short answer
to "What is epub3?" is, XHTML5. This means that more types
of media can be in an ebook. (Finally, my .gif image can come
to life!)
I have not started reading, "Accessible Epub3" yet. It has 104
pages.
O'Reilly has the books available in the following formats: DAISY, ePub,
Mobi, PDF.
ALERT: To get the freebies from O'Rielly you have to establish an
account with your email, address, and phone number. If you sign
in using your Twitter account, they (currently) use READ access.
These book are also free downloads at Kobo books:
What Is Epub3
https://www.kobo.com/us/en/ebook/what-is-epub-3
Accessible Epub3
https://www.kobo.com/us/en/ebook/accessible-epub-3
Available on a kobo reader and
"Download options: EPUB 2 (DRM-Free)"
Be it known, both books are now available at Amazon as a free Kindle download.