TCP/IP Illustrated Volume 1: The Protocols. It changed my life because I realised I could better comprehend systems others had already pored over and offer new insights. By comparing the contents of this book to RFCs I was able to propose numerous new remote operating system detection strategies which I published. I then had contact from a hacker group on the other side of the world, plus web visits from a swathe of militaries globally. (This was back in the mid-late 1990s when people didn't bother obfuscating their IP.) Gave me the confidence to go do my own thing in a lot of ways, including leaving Australia for its lack of R&D opportunities outside of military/academia, and probably a significant historical factor in my current position as a founder.