Reddit mentions of Power Pivot and Power BI: The Excel User's Guide to DAX, Power Query, Power BI & Power Pivot in Excel 2010-2016

For DAX, SQL BI is a great resource (especially free articles).

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This book helped me learn powerpivot/DAX when I was first starting out:

https://www.amazon.com/Power-Pivot-BI-Excel-2010-2016-ebook/dp/B019BNG4YW

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I can't really speak for courses since I've never taken one. I mainly scour BI blogs, forums and buy books, but these guys seem to know what they are talking about:

https://pragmaticworks.com/Training/On-Demand-Training/Introduction-to-SSAS-Tabular

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I have been working with Power BI for the last 2 or so years since it was released. So most of what I have learned has been either directly from microsoft or figured out on my own. I would Echo Data_Cruncher's statement below. In order to match your vision of what you want to display with the reality of your actual data, you should get familiar with DAX.

Good DAX books I have read:
Learn to write DAX from Matt Allington (good book, lots of basic concepts)
https://www.amazon.com/Learn-Write-DAX-practical-learning/dp/1615470417

Power Pivot Power BI by Rob Collie (funny book with more advance concepts)
https://www.amazon.com/Power-Pivot-BI-Excel-2010-2016-ebook/dp/B019BNG4YW/ref=mt_kindle?_encoding=UTF8&me=

Also, since I figured if I was trying to learn others would want to know what I have developed. So I have built a website with short tutorials on how to use and manipulate data in Power BI. You can access it by going to www.powerbi.tips.

As a final thought you should also check out he Power BI youtube channel. Start at the beginning at Video 0-1 and work your way through the tutorials. It starts with some over views and then quickly gets into how to build dashboards.
https://www.youtube.com/playlist?list=PL1N57mwBHtN0JFoKSR0n-tBkUJHeMP2cP