I am currently reading the first chapters of a Powershell book called "Windows Powershell in Action" written by Bruce Payette.
There are currently Five chapters available and I am done with the first two. It's very well written with lot's of examples and a good explanation of how Powershell works and what the different parts means. I like the fact that in Powershell you can create aliases of commands which can make a script more- or not readable at all. So be careful when using aliases or describe your aliases in comment within your script, you will remember them tomorrow but maybe not next month.
I will definitively continue reading the next chapters as I receive them.
Another very good Powershell blog is "/\/\o\/\/ PowerShelled" where he actually links to a video of the author of the book I am reading. He also wrote (11 now) different blogpost about Powershell and Active Directory, check it out very very cool.