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 (for now Varnish 3.x only)

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