If you're interrested on reading more about Pex, here are some online document that should get you satisfied:

• the tutorial: http://research.microsoft.com/pex/articles/pextutorial.pdf
• the reference manual: http://research.microsoft.com/pex/wiki/Reference%20Manual.html (single page)

The tutorial contains hands-on labs and exercises to understand how Pex generates new test cases. Highly recommend if you're interrested on Pex.