I bought this book on recommendation, because I was interested in a popular account of the Monster group. I've finally had to give up reading it, because it ultimately turns out to be a historical review of the figures involved in the long history of the mathematics of symmetry, with virtually no actual information about symmetry. All the mathematical examples are excessively verbose, making them largely impenetrable. In his attempt to "simplify" the mathematics, Marcus Du Sautoy has succeeded in removing enough information from the problems and solutions as to make them incomprehensible. At every turn, I felt that he was on the brink of giving me the information that I was looking for, only to have my hopes dashed at the start of the next section.
If you are interested in the characters in this story, this is a great book, but if you are interested in the mathematics, it's virtually worthless.