Take Control ebooks are having a 50% off sale of their ebooks until August 25. Most of the titles are about various Mac, OS X and iOS topics. (I'm not affiliated with them in any way, but have purchased numerous titles over the years, and been very satisfied with the content.)
No, it seems to subtract 50% automatically once you proceed to checkout (at least it just did for me). The code it applies is apparently "CPN40818BTSSALE", just in case it's not filled in for you.