A few months back I had a long talk with Sriram, on doing P2P for one of the .NET applications we were thinking about. We took a look at the MS P2P kit, but were disappointed as it was pure COM/C only, with no managed interface.
I got this book that talks about P2P with VB.NET. A nice and clear approach, the book starts introducing P2P and takes you through the different approaches you can take to create a P2P application. It also covers some of the most common P2P technologies available out there and also how you can do it from scratch with .NET.
I would have liked it better if the author had spent some more time with the Windows P2P kit.
If you are into P2P development and .NET, this book is of course a must read. It may even be a good read if you are working on networked applications, as there is a lot of information on networking and remoting with .NET in this book.