NB. I was sent this book as a review copy.
This is an extremely clearly, well-written book covering a lot of ground in the mathematics of cyphers. It starts from the very basics with simple transposition cyphers and goes all the way through to elliptic cyphers, public key cryptography and quantum cryptography. Each section gives detailed examples where you can follow precisely the mathematics of what underlies the encryption. Indeed the mathematics is non-trivial in a fair number of places, but it is always explained well, and I think that anyone with a first year university level of mathematics should be able to understand the bulk of it. I think that if you were to come at this book with a high-school level of mathematics, there would be some aspects which would be pretty hard work, but with some persistence, even those would be understandable, and perhaps the breakthroughs in understanding would feel like a great (though doable) achievement for the maths enthusiast.
The history of each type of encryption algorithm is discussed extensively, and the human stories absolutely add to the flow of the book. In addition, once the algorithms are discussed and examples are given, the pitfalls of each one is also presented, showing how would might find vulnerabilities in each of the crypto-systems.
Overall, I would say that along with The Code Book (which I seem to remember is somewhat less advanced), this would give a good grounding in both the history of, as well as the modern developments in cryptography, and would be an excellent companion to any university course in the subject.