Manning Publications have a great variety of books on .NET development. There’s the stuff you’d expect, such as books on C#, ASP.NET and SharePoint, but they’ve also produced books on IronPython and IronRuby (not many books on these languages), functional programming is both F# and C# and doing brownfield development and building DSLs using .NET. Many of their books have helped me get up to speed with .NET development, and I’m currently working my way through The Art of Unit Testing.

An opensource book on how to use jython 2.5. Made by the creators themselves.

A book on testing software at Microsoft! It’s a gift to anyone wanting to make a joke at the company's expense. Many a user would claim that, based on its products, there is no evidence that testing goes on at all and surely such a book would have very few pages. Of course this is unfair, but fun. Getting software right isn't easy and given the size of the Microsoft codebase you can expect one or two bugs to have slipped through. So a book on how the software giant tackles the problem is a good idea and should be instructive.