Windows PowerShell MVP Keith Hill's Blog is a great resource for learning PowerShell. He has written a series of posts titled "Effective PowerShell" and combined them into Effective Windows PowerShell: The Free eBook as well.

• Effective PowerShell Item 1: The Four Cmdlets That are the Keys to Finding Your Way Around PowerShell
• Effective PowerShell Item 2: Use the Objects Luke. Use the Objects!
• Effective PowerShell Item 3: Know Your Output Formatters
• Effective PowerShell Item 4: Commenting Out Lines in a Script File
• Effective PowerShell Item 5: Use Set-PSDebug -Strict In Your Scripts – Religiously
• Effective PowerShell Item 6: Know What Objects Are Flowing Down the Pipe
• Effective PowerShell Item 7: Understanding "Output"
• Effective PowerShell Item 8: Output Cardinality – Scalars, Collections and Empty Sets – Oh My!
• Effective PowerShell Item 9: Regular Expressions – One of the Power Tools in PowerShell
• Effective PowerShell Item 10: Understanding PowerShell Parsing Modes
• Effective PowerShell Item 11: Understanding ByPropertyName Pipeline Bound Parameters
• Effective PowerShell Item 12: Understanding ByValue Pipeline Bound Parameters
• Effective PowerShell Item 13: Comparing Arrays in Windows PowerShell
• Effective PowerShell Item 14: Capturing All Output from a Script
• Effective PowerShell Item 15: Using the Output Field Separator $OFS
• Effective PowerShell Item 16: Dealing with Errors

I just ran across these today and I look forward to exploring each in the series.