It's literally the definitive guide on CSS, and frankly, it's the gold standard for any book calling itself a definitive guide. An inordinate amount of work went into the book's 1126 pages. You will learn something every time you open the book. It will pay for itself the next time you wonder, "how do I do X with css?", because you don't have to search. It's right there in the book.
If you do software as a profession, is the book really THAT expensive at £45? Having a deep understanding of CSS could make you significantly more than that.