The official documentation contains a install guide, a tutorial and a reference.
If you run one of these release, you should really upgrade:
Documentation for old version 3.0 (unsupported)
Documentation for older version 2.1 (unsupported)
The bleeding edge:
Documentation for the current development version (git master)
For the historically inclined, Archive.org has a complete copy of the old Trac Wiki.
The Varnish Book¶
A comprehensive user guide for System Administrators and Web Developers in book format provided by Varnish Software:
Online Reference Guide for version 3.0 (unsupported)
Get the book as a PDF download
The Varnish (Web Developer) Wiki¶
Varnish Software has compiled a wiki for Web Developers with Tutorials, Step-by-step guides, tips, VCL snippets and more. The content focused on the following web technologies:
There are also some generic resources available: