The Enigmail Handbook is a 123-page comprehensive document integrating detailed installation and usage information, technical references, FAQs, troubleshooting, and general tips and tricks.
You can download the original PDF or view the Handbook online:
The Handbook (English, PDF), Version 1.8
The Handbook (English, HTML), Version 1.9, converted to browsable pieces of HTML
The Handbook (German, PDF), Version 1.0.1
The current edition is aimed at Enigmail version 1.9 but contains also guidance on previous versions.