Transfer an ibook to an android tablet

The majority of ibooks have digital rights management on them that tie them to the iBooks app on iOS devices and Mac computers, it's generally only free ibooks (and publishers that don't include DRM e.g. O'Reilly) that can be read in other apps. An ibook's requirements are listed on its description page in the store.

If you want to be able to read ebooks on both your iPad and android tablet you should buy them from a store that has apps for both platforms.