The Dojo Web site hosts the official Dojo documentation. From Dojo's Web site you can browse or search the Dojo Toolkit API by version or post Dojo questions in the Dojo Community.
The Dojo site hosts an extensive set of tutorials that cover a range of topics. From introductory topics like getting started with Dojo, to more advanced topics on working with deferreds, most things Dojo are covered in a tutorial.
The "ArcGIS API for JavaScript" tag on the ArcGIS Blog is also a resource to watch as Esri employees regularly publish valuable JavaScript info there.