Another in O'Reilly's Hacks series - I think this is my favorite so far, probably because it has the most useful ones, the most new ones, and the most ones I knew about all in one place.
The book categorizes the hacks into suitable groups: Unix Host Security, Logging, Network Intrusion Detection, and others. All hacks are listed in the contents so it is easy to browse throug, either for something interesting or for something needed.
One of my personal disappointments is the entire chapter, plus a few other hacks, dealing with Windows. My Windows security consists of keeping the system turned off, or at least disconnected from the network, and both are rather effective. My *nix bias is showing.
Another disappointment is that many hacks have alternatives - some are explored, but many are just mentioned, or not even mentioned at all. I understand the limited space for the Hack books, but I think more could have been done in this area. For example, IPsec had hacks for Linux, freeBSD and openBSD - good. Secure IMAP only mentioned UW-IMAP, and I'm willing to bet that Courier-IMAP is even better, and I suspect there are others. The same could be said about securing BIND - djbdns was not mentioned at all.
Still, I think this Hack book has a lot of meat, and most of what it contains should be useful for everyone.