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I'm a big fan of Pester. It is a fantastic testing framework, that allows you to write PowerShell tests in a way that perfectly suits the style of the language. However, like many open-source projects, the documentation is a little lacking. This book more than makes up for that. Starting with the basics, the book introduces you to all the major features of the framework. It then moves on to more complicated topics, building on your knowledge as it goes. Highly recommended.