That doesn't fair well for foreigners. I have a temptation to just go with Du for everything and speak to everyone like a 'mate'. Kind of like how in England even the police say "Can you stop there, mate, I need to search you for drugs, mate". How rude, but it happens. I've so far been trying to stay with Sie, but that's tough, as it feels strange saying "möchten Sie ein Kaffee?" and a little later saying "Meine Mutter, sie hat blau Augen" for example (excuse the typos)
The plural of "Buch" is "Bücher". "Büchen" doesn't exist.