Which profile is used for a book, and what does "default" mean?

Every book has an author profile. New books use the profile marked "default" unless you pick a different one in the wizard or in the import screen; books created before version 1.9.6 use the default profile as well. To change a book's profile, open its workspace and choose another one under Book details: the author name and publisher are refilled from the chosen profile and you can still edit them. To make a different profile the default for new books, open the Authors page, select it and click "Make default". A profile that books still use cannot be deleted; switch those books to another profile first.