My girlfriend and I occasionally share books (usually she borrows mine), but my understanding is that the books you "buy" are licensed only to a single device, and you can only share a book one time, and even then only if the publisher allows it.
I could be misunderstanding things, but it seems like if she wanted to borrow one of my books, she would probably need to borrow my Kindle to do it, and essentially borrow all of my books to read one.
Until things get sorted out, I think I'll pass on the eReaders.