When you open a book in Adobe Digital Editions, you get an error stating that another user account has the book locked:
Check the Adobe ID used to currently authorize Adobe Digital Editions. If it is correct, contact the e-book provider to have the activation reset for the e-book. After the activation is reset, download the e-book again and it is associated with the account activated in Adobe Digital Editions.
The check the Adobe ID currently used to authorize Adobe Digital Editions, choose Library > Authorize Computer.
This error happens because the book is assigned to another user or something has gone wrong with the migration from anonymous activation to named activation.