Also see IronPython.info for more information about various usages of IronPython.
Since IronPython is a implementation of Python 2.6, any Python documentation is useful when using IronPython.
IronPython's sweet-spot is being able to use the .NET framework APIs directly from Python.
IronPython also runs in all major web-browsers through Silverlight, which contains a significantly large subset of the .NET framework APIs.
When all else fails, you can always look at the IronPython source code
Regression Tests - List of regression tests currently passing