If it's an "issue" that might require changes in the book - I'd enter it into JIRA. For general discussions, you can use Confluence, or JavaLobby's Forums. There's also the Equinox Mailing List.

For Spring Development related questions, outside of the book, I encourage you to use the JavaLobby's Forums or the Spring Forums.

Of course, you can always e-mail me with any questions too.