I know that with WordPress For Dummies which is published under the same company, Lisa is able to provide updates in PDF format that are available for free from her site and the Dummies.com site while every 18-24 months, she writes a new revision of the entire book to bring it all up to date.

I don't see the WordPress bible being able to go down the same route due to the length of the book but who knows. If it contains most of the guts of WordPress, I don't see that changing much due to backwards compatibility so I think the book will be a valuable resource for a long time.