The words "hack" and "hacker" are thrown around quite a bit today. It seems a "hack" can be something as simple as using an AppleScript to auto-complete a task, which by extension would make that person a hacker. The traditional, or some would say true meaning of hacker is someone who gets to the very base levels of a piece of hardware or software, and makes it do things it was never intended to do.
Given the confusion around the term, one could be forgiven for picking up The Mac Hacker's Handbook, with the assumption that it would provide fun little tips/tricks to improve your jolly computing tasks. That assumption would be completely wrong. The Mac Hacker's Handbook is all about looking at the root level of the Mac OS, and exposes the various security issues and concerns, from open ports, to memory overflows. In short, if the thought of opening the terminal window frightens you, stay far away from this book.
If you like to get "under the hood," whether your intentions are to learn more about OS X security, or more nefarious purposes, this book is a treasure trove. I am not a "hacker" myself, but I know enough about the content to be concerned about the security of my Mac.
Having said all of that, this is, after all, a used book review, so this book focuses on exploits to the Leopard operating system. While many of the security issues have been addressed with Snow Leopard, that by no means renders the book useless.
If you are a hacker in the traditional sense, and you understand source code, stack heaps, and sandboxes, then this is a solid reference guide, especially for those coming from Windows and Linux.