This book is meant to be written in an open-source manner and the reason for writing it may be found here.
This book is written using Asciidoctor for it offers:
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a text-based document format, which eases collaboration and change tracking on GitHub
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OS independence
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PDF, DocBook, HTML5 support
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sub-document inclusion
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a rich set of tools like:
You can find an Asciidoctor quick reference here.