I'd like to add the following great (free) DAX course from EnterpriseDNA (not affiliated)DAX fridays with Ruth from Curbal.com is also a great learning source.DAX Patterns is a list of common patterns to solve common problems.When it comes to books I would like to recommend 'Supercharge Power BI: Power BI Is Better When You Learn to Write DAX' by Matt Allington. It contains a lot of exercises that will help you solidify your learning. It is a great introduction to DAX.The Definitive Guide to DAX is the DAX bible.

1. Sqlbi Dax
2. Dax Tutorial

Apparently a new edition is coming out in February 2019.

Sqlbi Dax

Dax Tutorial

Microsoft PowerPivot is a free add-on to Excel from Microsoft that allows users to produce new kinds of reports and analyses that were simply impossible before, and this book is the first to tackle DAX formulas, the core capability of PowerPivot, from the perspective of the Excel audience. Written by the world’s foremost PowerPivot blogger and practitioner, the book’s concepts and approach are introduced in a simple, step-by-step manner tailored to the learning style of Excel users everywhere.