Amazon just released a new version of its comprehensive standards guide for Kindle ebooks (PDF).
The new edition features a new rule about cover images and a list of fonts that should not be embedded in a Kindle ebook (because they are system fonts). I have also heard on Twitter that the new guidelines explains how to create pop-up footnote references.
- Updated 4.1 Marketing Cover Image is Mandatory: To provide consistency with Kindle Direct Publishing guidance, changed the 2700 x 1688 pixels recommendation to 2560 x 1600 pixels and changed the 5MB file size limit to 50MB.
- Updated 9.3.4 Avoid Using Fixed Values for Most Elements: Removed references to line-height.
- Updated 9.3.8 Using Embedded Fonts: Added list of Kindle fonts that should not be embedded in eBooks.