The Codebreakers
David Kahn's 1967 book is still the one people point to when they ask where the history of cryptography starts.
The Codebreakers by David Kahn appeared in 1967 and follows cryptography from Egyptian hieroglyphs and Caesar up to the Second World War. Kahn was a journalist, not a cryptologist, and it shows in a good way: the book is written for the reader who wants to understand why a cipher held or failed, not for someone who already knows the mathematics. It is long and loses some momentum in the later chapters, but it remains the best entry point into the subject.