The Browser Hacker's Handbook. http://smile.amazon.com/Browser-Hackers-Handbook-Wade-Alcorn/dp/1118662091
If people don't mention that, they probably couldn't get into a modern computer network if they tried. This book is fundamental to understanding the modern landscape of hacking and to understand the future of it.
The user is the browser, now. It's almost a whole new OS with applications and environments, controls of its own and I almost never see people mention it. Maybe because it's brand new, all the more reason to get it.
If you actually want to know how to exploit a system from the outside-in, get this book. If you want a power trip and to read code you'll likely never use outside of your own lab all day, get something else.