Required Reading: Optimizing WPF Application Performance
If you are a WPF developer, you must read Optimizing WPF Application Performance. It is chockful of tips, tricks and information about the inner workings of WPF. I've read it multiple times and I always learn something new. It is a glimse into the WPF source code, which you are building on top of whenever you build a WPF application. With a platform as vast as WPF which does so much on your behalf, you need to understand in fact what it is doing and how it is doing it. Don't read this paper before you've hit the curve, but once you have your bearings with WPF, you have to read it. Twice. Or more.