This book, "Drupal 7 – the Essentials" was written and published in 2011 for the DrupalCon London conference, and added under creative commons license to drupal.org in May 2012.
You are encouraged to edit and improve the pages in this book, correcting typos or updating outdated information and adding new things that fit in the scope of this book.
An epub version of this book is available at archive.org. It represents the online book as it was September 17 2014.
The initial version of this section of the Community Documentation came from the book Drupal 7: The Essentials, courtesy of NodeOne and Johan Falk.