Power Query is one of the newer parts of the Power BI offering. I haven’t been able to find any books (yet) about it so the web is the best bet for learning about it now. I added a sub-page of my Microsoft BI Resrouces page specifically for Power Query. I’ll be adding links to what I find related to Power Query. Apart from the documentation from Microsoft, I found that Chris Webb’s blog has a number of posts related to Power Query.