Autodesk Developer Network 2011
Autodesk
§Read once
§Getting Started with the Revit API.doc
§Revit Platform API Changes and Additions.doc
§Keep at hand always
§Revit API Developer Guide.pdf
§RevitAPI.chm
§What's New section is similar to Changes and Additions doc
§Read if needed
§RevitAddInUtility.chm – installer
§Revit Server SDK – file access on server
§REX SDK – Revit extensions framework
§Point Clouds Indexer  API
§Important utilities
§Add-In Manager
§RevitLookup
§VSTA Samples
§Samples
§
§    Revit Wiki Help:  http://autodesk.com/revitapi-wikihelp  new! in 2012
§
SDK Documentation
Here are the top level contents of the SDK.

For getting started, we recommend you read through the Getting Started With Revit API document and also can look at the Revit Platform API changes and additions document to get a snapshot of what has changed in Revit 2012 as compared to the previous release.

The Developer Guide is very comprehensive pdf document with focus on each API topic split across various lessons. The Revit API chm file is another great resource where you can search for specific APIs. The Whats New Section in this file highlights the new APIs in Revit 2012.

Besides this, as shown in the slide, there are a number of other resources specific to certain APIs which can be of great help.

The Revit SDK samples provide a huge knowledgebase on how to address specific programming tasks using the Revit API. The API documentation in the help file lists all the classes and their methods and properties, and the developer guide and samples explain and demonstrate how they work together to solve specific tasks.

With Revit 2012, now the product and API help system has taken the next step by moving to a online wiki-based system which provides support for a full community learning experience.