PGP (Pretty Good Privacy) is a computer program for the encryption of data and electronic mail, a powerful "envelope" that allows individuals the same privacy in their communications as enjoyed by governments and large corporations. PGP, which is freely available on the Internet, uses public-key cryptography - specifically the RSA algorithm, which is particularly well-suited to the needs of computer-mediated communications. This book contains a formatted vesion of the complete source code for the latest release (2.6.2) of PGP.
The original intent of this book was to defeat U.S. export laws (International Traffic in Arms Regulations). By putting PGP v2.6.2 in printed form (free speech and all that), it could then be shipped legally overseas to be scanned into computers and compiled. Or at least, provide plausible deniability that this happened.
There's even a page at the start with instructions on how to do OCR. Of course, it only takes one person to do this, which is why this 907 paged book probably went out of print so quickly. Now, it's just a bit of history.