This is the API documentation for Eigen3. You can download it as a tgz archive for offline reading.
Then, the quick reference pages give you a quite complete description of the API in a very condensed format that is specially useful to recall the syntax of a particular feature, or to have a quick look at the API. They currently cover the two following feature sets, and more will come in the future:
You're a MatLab user? There is also a short ASCII reference with Matlab translations.
The main documentation is organized into chapters covering different domains of features. They are themselves composed of user manual pages describing the different features in a comprehensive way, and reference pages that gives you access to the API documentation through the related Eigen's modules and classes.
Under the Extending/Customizing Eigen section, you will find discussions and examples on extending Eigen's features and supporting custom scalar types.
Under the General topics section, you will find documentation on more general topics such as preprocessor directives, controlling assertions, multi-threading, MKL support, some Eigen's internal insights, and much more...
Finally, do not miss the search engine, useful to quickly get to the documentation of a given class or function.
Want more? Checkout the unsupported modules documentation.