I'm not sure why the books would disappear. DRM-locked books might be unreadable if the previous owner deregistered the device, but that wouldn't affect public domain books like the Project Gutenberg ones.
Anyway, yes, the 505 shows up as a mass storage device (a drive, that is) in Windows. I'm not sure about other platforms, sorry. You can copy your books like any other file.
As dreams said, don't worry about public domain books; you can pick up PD books in a jillion places. There's a list of them around here somewhere.
Personally, my preferred public domain ebook archives (in order of preference) are MobileRead, Manybooks, Feedbooks, Project Gutenberg, and everybody else. For free non-PD books, the Baen Free Library and the Baen CDs rock. Hang around here for a while and everybody will give you conflicting advice on the best places to get books.
