My wife and I each have a 505, and I have made the following "discovery" about sharing library books:
We have installed Adobe Digital Editions onto each computer, and activated both computers and both 505's with the same Adobe ID.
If either of us goes to the library and checks out a book, it is assigned to the computer that checked it out. If I try to follow the ADE directions for copying files to other computers the book appears in the library, but when I try to read it on the second computer, I get a message that says there is no borrowing record, and it will not open.
However, if I connect each reader (one at a time) to the computer that did check out the book, I can copy the book to both readers.