The official documentation contains a install guide, a tutorial and a reference.

We're trying to keep the official documentation in sync with the code. Both are kept in same source repository and both are available online:

You also probably want to have a look at the wiki - especially the VCL Examples

Quick install guides various operating systems:

The Varnish Book

A comprehensive user guide for System Administrators and Web Developers in book format provided by Varnish Software:

If you have feedback or want to make improvements to the book please do so by filling issues and/or send us pull-requests.