The complete plugin user manual is available online. However this online version is missing the EclipseHelp framework and will look different than the version included in the plugin.
The user manual also has a short tutorial on how to create and build a simple avr project. You can read it online or, once the plugin has been installed, via Help > Help Contents -> AVR Plugin -> Getting started.
If you already use the plugin you might be interested in what is new for the latest version.
- The user manual has a nice overview with regard to the changes of the currently released version.
- The Releasenotes contain an overview of the changes for the last stable release.
- The Changelog contains the detailed list of all new features, changed features and bugfixes for the current release (incl. beta releases)