Linux Server Hacks is well worth the money (only $15.50 from Bookpool). I am a big fan of these "cookbook" style books, I find the short explanation coupled with good examples to work really well. It's also a good starting point on a wide range of topics, the tips make an excellent introduction to a topic and you feel confident about delving deeper.
A good example is tip 66, "Quick logins with ssh Client Keys", before this I was unable to get this to work. I had tried a couple of methods suggested via google, but I just couldn't pull it off. A couple of paragraphs and three commands later I had things working. The same with tip 38, "Using rsync over ssh", this short tip was all I needed to get started and a lot more user friendly than the over 40 screen-fulls you get with "man rsync".
The "hacks" format is one that I like and I am looking forward to more titles in this series.