Like me you may have seen updates available for some of your iBooks, and that might have made you assume that when a book gets changed in the Store, it gets updated in your Library. This appears not to be the case. I’m not sure why some books do get updated that way; but not all do.
Apple's own documentation for the Swift language, for example, had changed since I originally purchased the iBook; but no update was produced in the Store (causing me to rely on out of date information) and the only thing I could do was to delete and re-download the up-to-date iBook (which removed all my annotations, of course).
Is there some mechanism that I'm missing for ensuring that iBooks in my Library are up-to-date?