I have been reading IronPython in Action over the last few days and enjoying it, even though I'm not much of a .NET developer at the moment. Its main strength is the down-in-the-details treatment of building real .NET apps. It covers the basics well but does not shy away from more advanced stuff, both at a language level - like meta-programming - and at a .NET level, like DLR/Silverlight.
I don't know what my next big Windows app will be but I'm pretty sure it will be developed in IronPython with this book open beside me. Recommended.