[Aleron Ives]

Yes, but you need to be careful with library books, since you don't have the right to keep them indefinitely, and adding them to your Calibre library may be a recipe for confusing them with the books you own.

Furthermore, some libraries require you to return loans from ADE, so if the Calibre plugin doesn't support that, then you have no way to return a loaned book after you finish it so that other patrons can read it sooner. You'll just have to wait for the loan period to expire before the next person in line can get it.

*edit*

After doing a little reading, it appears the plugin does support returning library books. Huzzah!