I just finished reading the latest version of PowerShell Unleashed last night. Although there are some really great chapters (like the discussion on security), I don't recommend this book as your first introduction to PowerShell. To be fair, the authors don't pitch it that way at all, but I think if you dip in to PowerShell and then read this book you'll be a bit over your head quickly. It's a multi-author book, which is problematic anyway - and you can hear each author distinctly in each chapter. For instance, one author explains concepts using Perl as a guide; another uses vbscript and WSH. Some of the chapters feel like they were written in total isolation from the others.
Still, I would recommend grabbing a copy to read, at least for the chapters that you need at a given moment. That "isolation" factor at least provides the ability to do that.