Ron, are you mad? He'd work out in two minutes it came from the Black house. No one was able to get in here after old Mrs Black died, except for Sirius, and then once he DID get in he called up his friends and they cast the Fidelius (this was all part of the lesson this morning on the Fidelius Charm, weren't you paying attention?)

You don't think it would be slightly suspicious if we turned up in his office with a book that came from THIS PARTICULAR HOUSE?