Hi, Rachel. Are you on a Mac? ADE for Mac will not recognize the PRS-300. In which case you have to use the Sony software to transfer library books. It sounds like you got a bad reinstall of the Reader Library. It is not a simple uninstall/reinstall process with the Sony software.
First I would try Post #8 at this thread:
https://www.mobileread.com/forums/showthread.php?t=59980
If that doesn't work then I would do another reinstall of the Reader Library. If you are on a Mac you need to follow Post #5 on this thread:
https://www.mobileread.com/forums/showthread.php?t=78921
If you are on a PC, then maybe someone else will post some other ideas.