Jon Skeet has an entire chapter dedicated to them in the 2nd edition of C# in Depth. It's roughly 40 pages based on the last MEAP update. This isn't final since, as Joel mentioned, it's not out yet, but you could buy it and get access to the MEAP releases: http://www.manning.com/skeet2/
C# 4.0 in a Nutshell covers it in Ch. 13, "Diagnostics and Code Contracts." Based on the chapter headings in the table of contents I estimate ~17 pages on the topic.
I imagine Andrew Troelsen's Pro C# 2010 and the .NET 4 Platform book will cover it, but I wasn't able to find a TOC for it.