There aren't many good maths books out there, but this is definitely one of them. It manages to describe the concepts - mentioned below in other reviews - so well that by the end you wish you could meet the author and have a chat about mathematics; the sign of a really good book.
The level is about 18+ and it will be of great benefit to maths students going to or at university. It was recommended to me by my lecturer; not surprisingly, I ignored him, but I found it a few years later. I kicked myself when i finished...I wish i had read it earlier. The title doesn't help either...it turns you off immediately...so he's either brave or stupid to call it that...!
It is true that e is an extremely important number, and really, it is far more interesting than pi in many ways. Unravelling its history leads to an explanation of many interesting areas of mathematics, and calculus is described well. The explanation of logs wasn't all that great, but it tied the book together.
If you're a maths student, it will help give subjects you cover some background and perspective. You may understand them better too, so...go and read it now!