I'm working on a large iBooks Author file. About 100 pages in, when I went to save it, it said Document Could Not Be Saved. I don't know if it is because of the size of the document, the shape that I added to create a mask over text, or the odd paragraph styled that I redefined.
There isn't a Save As feature and you can't export to an .iba file.
It turns out that File > Duplicate is the same as Save As, but that isn't straight forward. I was able to Undo for several steps and then save it.
actually, Save As... does exist. Anyhow, you should be careful with using Duplicate or Save As... with iBooks Author. Each book has a auto-generated ID which iBooks uses to keep track of it. If you use Duplicate, you get another copy of the same ID. Try putting both of these in iBooks and you will find that tapping on one will open up the other. This can get very confusing very quickly if you are using this method for new drafts or sharing docs with other authors.
To prevent this bizarre functionality and ensure that you actually create a new book, hold down the Alt key when you click on the file menu, you'll see other options appear including Save As...
Ignore Save As... and use Duplicate as New Book. A new book will appear in front of the old one. Simply type the new file name and hit enter and you'll have what you want. The old book will still be open behind your new one.
