The Code Book by Simon Singh (1999)

Another book I pulled off my shelf recently to give to my kids to read (both teens who are into science)—this is a terrific and thoroughly readable history of secret writing and cryptography, starting from ancient Egypt and going through quantum computers (at least as they stood as of 1999, obviously). A friend of mine got this right after reading Neal Stephenson's Cryptonomicon and lent it to me, and I was so into it that I read the entire thing in one day and then just bought my own copy.