Though not really a book, the official documentation downloadable with the Code Contracts software is pretty good.
For specific questions, you can use the official Code Contracts forum. The developers seem to be pretty responsive.
By the way:
My understanding is that Code Contracts are not part of C# 4.0. Rather, they are part of the .NET 4.0 BCL and available as a separate assembly for .NET 3.5. Both .NET 4.0 and 3.5 requiere you to install additional software to use the contracts.
