If you upgraded to Mavericks, and noticed that not all of your books were migrated from iTunes into iBooks v1.0, update to iBooks 1.0.1 to import the rest of your books.
After entering in an Administrator name and password for the OS X Mavericks installer you see the following message: “This copy of the Install OS X Mavericks application can’t be verified. It may have been corrupted or tampered with during downloading.”