The Varnish documentation consists of three main documents:
After The Big Varnish Picture, this Users Guide is organized in sections following the major interfaces to Varnish as a service:
Starting and running Varnish is about getting Varnish configured, with respect to storage, sockets, security and how you can control and communicate with Varnish once it is running.
VCL - Varnish Configuration Language is about getting Varnish to handle the HTTP requests the way you want, what to cache, how to cache it, modifying HTTP headers etc. etc.
users_report explains how you can monitor what Varnish does, from a transactional level to aggregating statistics.
users_performance is about tuning your website with Varnish.
Troubleshooting Varnish is for locating and fixing common issues with Varnish.