This is the best short survey of mathematics I know of, and I think the format works very well (4 pages each for 50 key mathematical ideas). I personally read one idea per day for 50 days, and this book delivered a bright spot for every one of those days.
To address another reviewer's comments, I do agree that this book has lower and upper bounds to be aware of.
The lower bound is that readers should come to the book with at least a general familiarity with the subject of mathematics, including having at least heard of concepts like complex numbers, calculus, probability, chaos, abstract algebra, group theory, Fermat's last theorem, the Riemann hypothesis, etc. And certainly readers should come to the book with a genuine interest in mathematics. In other words, this isn't a book for readers with no background or interest in mathematics, nor readers with a fear of mathematics.
The upper bound is that the book doesn't (and can't) develop the mathematical ideas in step-by-step detail. Rather, the book goes into just enough detail to give a meaningful sense of what the ideas are about, and it does this quite well, with nice features like timelines, examples, historical asides, etc. This book isn't a mathematics textbook, nor does it purport to be, so it shouldn't be judged on that basis.
The only thing I really found lacking was that the book doesn't include suggestions for further reading. But this omission isn't enough to lower my rating from 5 stars, and I highly recommend the book to anyone looking for a short survey of mathematics. Tony Crilly provides a wonderful and panoramic guided tour of the subject, spanning from elementary to fairly advanced ideas, and does it in a way that both entertains and reveals the rich beauty of mathematics. For readers who take the tour and feel sorry to see it end, I suggest moving next to the massive and outstanding The Princeton Companion to Mathematics.