Windows Powershell in Action by Bruce Payette, who designed Powershell, read both the 2nd and 3rd editions. Powershell is only 80% documented otherwise. Although before that I was watching Jeffrey Snover videos, and using his idea of fun and exploration.
I’m planning on creating a series of powershell videos to teach people how to use powershell. That’s similar to how I learned powershell, by watching videos and then trying the things I saw.
Write Powershell code every single day. View Powershell information on commands and start with the basics and move on to more complicated commands. It is like any other langauge, you must immerse yourself into it. Find some processes you often do and try to automate these processes using PS. You will be suprised on how much you will learn. I have been using for over 10 years, and I still learn something new all the time. I still have to google and use get-help (your friend) to re-learn the syntax of certain commands. Just keep pounding away at the keyboard. I use it everyday and perform all kinds of processes from utilities to installing programs, to picking lottery numbers Wrong lottery numbers. Persistence is the key!!