Of the two books, I (wsy) recommend both enthusiastically; I'd say if you're more of a “I need to have deep knowledge of how does this actually work” person, go with Zdziarski first and askins/Nielsen second, and if you're more into “I'm a sysadmin, how do I hook this up in my server room”, go with Haskins/Nielsen first and Zdziarski second. The two books overlap about 50%, so it's worth reading both of them. Of course, that's just my (wsy) opinion, for what it's worth.

And of course the official guide: