This books doesn’t require a lot of comments. It’s the DAX Bible. The Definitive Guide to DAX means what it says – it’s The Definitive Guide to DAX. You’ll find (almost) everything about DAX in this book. It’s not a book for fast reading. This book will stay with you for a while. Use this book to read about DAX, to understand DAX, to practice your DAX (using companion content, which includes a demo database), to optimize your data models and DAX. The book is not easy reading, because DAX is not just a list of functions, DAX is based on the fundamental concepts we need to understand. To understand the concepts of DAX you’ll need help of an expert. Marco Russo and Alberto Ferrari are the best DAX experts and their consulting time is really expensive. But they put all their knowledge into this book which will cost you less than $40-50. If you’re going to work with DAX you must have this book.
Buy the book: Definitive Guide to DAX, The: Business intelligence for Microsoft Power BI, SQL Server Analysis Services, and Excel, 2nd Edition (Amazon.co.uk link, I’m in Europe and this where I got the book. I believe you can find it on amazom.com as well). I left the same review on amazon.co.uk. It’s just my opinion as is about the book that I bought and read, I’m not affiliated with the author, the publisher or Amazon in any way.