How do convert a no DRM book from iBooks to ePub?

I downloaded a DRM free technical paper, Human Interface Guidelines, from iBooks. It has an epub suffix and is actually a directory with subdirectories named OEBPS and META-INF.

I want to read this on my Android tablet. There's no DRM, but I can't convert it to ePub. Pdf would be fine too. I tried zipping it up and converting it in Calibre, but that gave an error.

I also tried opening it up in iBooks Author but it doesn't recognize the format.

Any clues? Thanks in advance.