iBooks Author 2.6
Apple has updated iBooks Author to version 2.6 with the capability to add images and videos from the Photos app using the Media Browser or via drag and drop. The ebook production and publishing app also gains support for wide color gamut images and Apple’s standard unspecified performance and stability improvements. (Free from the Mac App Store, 409 MB, 10.11+)
Just tried it.
You can import epub books. I tried and it looks very pretty. but using one of the furnished templates, a 371 KB document is over 30MB in the new format!