On Amazon the paper price and the Kindle price are about the same... ~$25 i think it is a dollar more for the privilege of not carrying 4.2 pounds around!
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This book focuses on long-lived fundamentals; on helping readers learn how to think about solving problems and taking on new challenges; and on tools that don't change with the Drupal version.
That said, the book is not already outdated nor will be soon in the many version-specific tech details it does include; unlike many other books available it waited until Drupal 7 came out and it even was updated in the last hour to reflect the latest Views 3 user interface. This book is a solid foundation for years to come.
Disclosure: I'm an extremely biased source, and almost as familiar with the book as reviewer Michael Ross. Wow, i wish he had been the copy editor.