All the code shown or mentioned in the book is (or will be) linked to here.
Site code
https://github.com/definitivedrupal
This is the entire history of the definitivedrupal.org web site (not a model for project management, plus it started before Drupal 7 was stable).
A sanitized (personal user data removed) database dump is coming.
Contributed modules
Modules which were made in the book and live on as contributed modules on Drupal.org: