Less is a compiler for CSS. Why do I feel I need to start a review with that? Because this book focuses a lot more on CSS than Less. Actually, I may have learnt more about CSS than what it was expected to learn. There is nothing wrong talking more about the base technology when the piece you're talking about is built on it, but when you talk a lot more about the first and not the second, you have to seriously reconsider your book title.
Another thing that annoys me is when the author keeps talking about the examples outside the book. "Check the examples which you can download in the previous URL" is lazy explanation. Instead of building knowledge blocks and then expand over them, the book uses the lazy escape of not telling you anything and expecting yo to read things somewhere else.
In the end, it's a nice book to learn a thing or two about modern CSS, but barely something good about Less.