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. For now it covers Varnish 3.0. It is available:

A version for Varnish 4.0 is on the works and you can see the branch on github. Please provide feedback by filling issues and/or send us pull-requests. Please use the Varnish-Book-v4 label.