A foreword from Salvatore La Terra, Textbook project manager:
Whenever a potential candidate approached me and told me that he wanted to become a Magic judge, I always struggled to point him towards a suitable resource.
The Comprehensive Rules are very bulky and difficult to read for candidates. When I pointed them to a massively and overly technical document written in a foreign language, I knew I would not see them again soon, if ever.
On the other hand, when I suggested them to study on the nimble Basic Rulebook provided (and translated) by Wizards for starting players, I had them fail miserably on the written test.
I felt the program needed a document covering the middle ground between these. A document that is friendly, inviting to read and easy to grasp, but at the same time teaches aspiring judges all the rules they will need in their activity within stores.
As I could not find this document anywhere in the judge program, I did what any sane person would have avoided at all costs: I decided to write it. Of course, I couldn't have done such a massive deed by myself, so I asked for the help of my fellow judges: tens of extremely talented judges volunteered some work on this project. To this date, if you want to give us a hand with maintaining, expanding and translating the Textbook you can hop on its Judge Apps page.
Thanks to them for all the time they've spent to bring us the Judge Candidate Textbook; and thanks to you, dear reader, for spending your time becoming a better Magic judge.