I'm going to do my best to avoid making this blog look like the Yellowpages but I came across this blog today that is pretty stellar. DP does an amazing job of giving in-depth advice without using a wall of text (something I am very bad at). There are sections on there about cast rotations, specs, healing, Shadow Priest dps, gems, gear, PVP advice against every class and how to overcome them & more. Just overall an awesome blog that any Priest should read.
Even at level 27 I found info on there worth taking notes on and it really gave me an idea of what to prepare for.