I have recently become obsessed by the Microsoft .NET Micro Framework. The .Net Micro Framework is the perfect blend of tools and hardware for me, and I have jumped in head first.
I first picked up Jens Kühner's book Expert .NET Micro Framework, which is a great book: practical, to the point and not too verbose. I then skipped through Embedded Programming with the Microsoft .NET Micro Framework written by Donald Thompson, Rob S. Miles, which also has some useful information. If you're looking for a book to get into the framework I would recommend Jens Kühner's.
To get my feet wet, I bought the Digi JumpStart for Microsoft .NET Micro Framework kit, on the basis of Jens' excellent introduction to the available modules, and have been using it to experiment with the platform. I chose the Digi module because it is compact, cheap and simple.It includes a couple of GPIO's, an Ethernet port and a serial port, which will allow you a surprising amount of connectivity. I had no idea how widespread RS232 serial still is after all these years. My very first experience with communications was with an RS232 2400 baud modem, and throughout my early professional life I have worked with serial devices such as VT terminals - I thought USB must surely have killed it off by now but it seems RS232 is far from dead, with devices all over the place supporting it. Using the Digi Connect ME / Wi-ME module you can hook up to everything from a ZigBee sensor network to a full robotic platform using RS232.
With a few small test projects under my belt I can say that the Microsoft .NET Micro Framework is living up to my expectations, and more. The framework libraries are concise, logical and well designed; the hardware integration has been easy to work with and the footprint is absolutely tiny for what they have packed in. I would recommend this platform to anyone building devices, especially where quality user interface and standard XML web services integration are factors in the design.