There is a review of HackNotes Linux and Unix Security portable reference on slashdot. Looks like a pretty good book, I know that the CodeNotes series are good concise references. They are typically under 250 pages (this one is 224 pages) which is nice when many books web developers are buying are thousands of pages.
I have CodeNotes for J2EE, CodeNotes for Java, CodeNotes for ASP.NET, and CodeNotes for XML. I have found all of them to be pretty good, the J2EE one has been most useful to me, and the Java one is good, but I know Java pretty well so I don't have much need for it. The books are also dirt cheap, so you can't go wrong.