Take Control has a 50% off sale on all Take Control Ebooks running until August 3.
I bought Take Control of Easy Mac Backups because I am an idiot and clicked the wrong checkbox (darn those similar book titles), Take Control of Mac OS X Backups (which is the one I really wanted because I have more advanced backup needs), and Take Control of Your 802.11n AirPort Network. That last book has proven immediately helpful by explaining some of the settings that I can usually find in a router's web interface, but Airport Utility helpfully obfuscates behind somewhat hidden menus and settings. Such as showing me where to adjust transmit power levels and explaining the differences between 802.11n channels (the latter is something I had no understanding of since they differed from the normal 1, 6, and 11 options provided by 2.4 GHz.)
I appreciate the hard work the Engsts (Adam and Tonya) put into supporting the Apple community, not only as the people behind Take Control, but also as publishers of TidBITS. I could have gotten the same information these books offer by trolling the web and assembling my own reference manuals, but paying less than $10 per book (PDF is the default format, but most books are also offered in a couple other formats) and having iTunes sync them to my iPod touch is even easier.