Chess Explained books provide an understanding of an opening and the middlegames to which it leads, enabling you to find the right moves and plans in your own games. It is as if you were sitting at the board with a chess coach answering your questions about the plans for both sides, the ideas behind particular moves, and what specific knowledge you need to have.
The Nimzo-Indian is one of the most important of all chess openings, and popular at all levels of play. Together with the Queen's Indian (also covered in this series), the Nimzo has provided the backbone of the opening repertoire of a large portion of the world's top players over the last 80 years. It is considered a fully reliable defence, and provides winning chances for both sides as it leads to structures of great strategic variety and complexity. Key battlegrounds in the Nimzo include the blockade, IQP positions, the handling of unbalanced pawn-structures, and the struggle between bishop and knight. All these topics are covered in depth in this book, and an understanding of them will prove valuable in a much broader context than just the Nimzo-Indian.
This is a "chess explained" book and not a repertoire book, but there's still so much good information packed in fewer than 120 pages. I am not a 1.d4 player, but I do play the Nimzo as black. There are a few critical suggestions that aren't covered, so if you are planning on using this as a resource to fight the Nimzo, it would be best as a supplement.