Microsoft Press is offering a free ebook on Power BI. And at 176 pages, it is not a lightweight book. Starting off with an introduction that covers the basics, it goes deeper into how to share the dashboard, refreshing the data, using Power BI Desktop and building a data model. It called Introducing Microsoft Power BI and its worth downloading and using as a daily reference. You can find it here: Free ebook: Introducing Microsoft Power BI.