The Epub3 spec is only few months old, and I’m sure there re many who re still learning its nooks and crannies. O’Reilly can help you with that.
They’ve just released a new free eBook with a collection of their best tips for how to make an Epub3 eBook. The book description has more detail: “Through practical tips and examples, Accessible EPUB 3 takes you inside the EPUB 3 format and explores how you can enrich and enhance content for all readers. Author Matt Garrish demystifies the process of making content easier to access, demonstrating how accessible practices are intertwined with standard content best practices.”
The eBook can be downloaded for free from O’Reilly in DRM-free Epub, Kindle, or PDF formats. Registration is required, but you won’t be asked for your credit card info.