Since I am the primary admin for over 25 OS X systems, it's important to me to have plenty of places to get quick answers.  Although books are quickly going the way of dinosaurs (at least those in printed form), I frequently get aggrevated at OS X articles on the Internet that don't mention the version being discussed.  Since one must seach on the version number which is hit or miss if the content states something like: "this worked in 10.4 and stopped working in 10.5, but is working again in 10.6."  Anyway, here's a list of Peachpit Press* books that are really pretty solid.