There aren’t many books out on PowerShell at the moment but I have been reviewing one that is coming out sometime in January. It is a book written by Bruce Payette who is the dev lead on the PowerShell team. The book is titled "Windows PowerShell in Action" (Manning Press).
From the drafts I have been reading this looks to be an excellent book. It’s primary goal is to cover using the PowerShell command line and writing scripts. It does a great and thorough job of covering the various nuances of using PowerShell and getting the most out of its scripting language. From where I sit, I give it two enthusiastic thumbs up!