I am thinking of moving my extensive eBook library to iBooks or Kindle. Has anyone done this, and what does it take?
I just don't want book libraries in multiple proprietary formats, and have to look for content across apps.
Open iBooks, drag & drop the book you want to convert to the Desktop. It will create an .epub file. Then open it with Calibre and use it to convert the book to the format you prefer, eg. AZW3 for the Kindle.
The way back you can imagine it, it is the same but in the opposite direction.
Note that there are now three main eBook formats
If the books do not have Digital Rights Management(DRM) then Kindle books can easily be converted to ePub as the format is similar, the reverse can also be done but you might lose some information as ePub data contains more things than Kindle data.
The easiest way to convert is to use Calibre which is a tool for managing ebooks including conversion, loading to devices and organisation of a library of books.
DRM books are tied to the application with which you bought them. An exception is that many ePub EBooks (but not iBooks and not Kindle) use Adobe's DRM and so can can be used in several devices.
There are plugins to Calibre, which are not legal in many countries, which allow removal of the DRM and thus use in Calibre allowing conversion between the formats. However I belive that there is no plugin for decryption of iBooks files.
Apple iBooks are very limited and therefor should be avoided if at all possible. Other eBook types are much more friendlier to use.