Is there any way to change the dictionary used in iBooks? When reading books written in a non-English language I would like to have access to the dictionary function in the book's language (not translation, just dictionary lookup). Currently I'm reading Don Quijote in Spanish, and it would be really helpful to be able to lookup unfamiliar words.
No, it's not possible yet, only English and Japanese as provided by Apple. Extra dictionaries are available from 3rd parties for the OS X version of Dictionary.app, but Apple does not provide any way to add these in the iOS version so far.