I just received the following great news of progress from our Content Publishing team. Make sure to check it out: 

The SDK team publishes updates to the existing Developer Documentation every month. Since the release of Dynamics AX4.0, they have added over 5,500 new topics to the online help available on MSDN. These new topics include information about classes and tables, creating and customizing forms, the unit test framework, the reverse engineering tool, the Code Access Permission framework, development best practices, and database objects.

You can find the new documentation in the Dynamics AX Library on MSDN, and you can download a list of the new topics from the Microsoft Dynamics AX 4.0 Documentation Updates web site.