At its core Crypto is a history of public key encryption present in the context of the struggle between individual desires, business needs, and national security. Public key encryption is a method for secure communications where neither party needs to send a secret key or code to the other. It represents a wild departure from classic cryptographic codes which require a separate secure method (such as hand carrying) to transfer secret keys before messages can be sent and received. Any secure credit card transaction you have done on the Net utilized public key encryption.
Levy presents more of a human history, concentrating particularly on Whit Diffie and Ron Riverest. Diffie (along with Stanford professor Martin Helman) filed the first patent for the idea of public key encryption. MIT professor Ron Riverest, along with Adi Shamir and Leonard Adleman patented the first practical method for public key encryption (RSA). All of them worked outside of government agencies. Levy discusses what motivated these individuals in addition to their actual work.
While the patents for public key encryption were not stopped by the National Security Agency (the government organization responsible for providing secure communications and who monitors ALL international communications), companies planning to export programs containing encryption faced a government crackdown. For example, Lotus needed secure communications for its Lotus Notes email program (which I use at Philips). This program was targeted at worldwide corporations who would not want business plans and information to be broadcast to potential competitors. However the government still treated cryptography as a military arm, just as it would a B-52 bomber or a nuclear-tipped cruise missile. In fact the government threatened to prevent Lotus from shipping Notes at all if they planned to sell it overseas. As time went on more and more companies had a need for secure communications. The explosion of the World Wide Web presented an unstoppable demand for secure communications to enable trade. The situation became insane with overseas companies selling cryptography-enabled products around the world including the US, but US companies could not sell equivalent products overseas.
In fact on September 18, 1998 your's truly became an international arms exporter, by sending cryptographic software overseas. The fact I merely transferred code from an overseas site to my machine and then back, did nothing to mitigate the possibility of my arrest by the FBI. Late in 1999, the government reluctantly relinquished restrictions on cryptographic exports, under massive pressure from businesses.
The book covers this struggle and many others in detail. I recommend Crypto to anyone with at least a passing interest in cryptography.