For a first contact with Eigen, the best place is to have a look at the \ref GettingStarted "tutorial". The \ref QuickRefPage "short reference" page gives you a quite complete description of the API in a very condensed format that is specially useful to recall the syntax of a particular features, or to have a quick look at the API. For Matlab users, there is also a <a href="AsciiQuickReference.txt">ASCII quick reference</a> with Matlab translations.