Qblade's author doesn't explain BEM theory - they assume you are already familiar with it. So you can experiment with Qblade for a while until the "plug-and-chug" method gets confusing or you just dive in with both feet and do the calculus!
Have I scared you yet!?
For general learning about wind turbines and how they perform, I think the general web encyclopedia is fine, especially since most folks don't really want the math they just want to know how variable A affects variable B. For real depth you have to go "off the grid" so to speak, by that I mean that the internet just doesn't cater to your needs. What you need are books and research papers. If you'd like to read up on the subject I can send you a lot of things that will lay it all out, and you can go as far into the mathematics as you like!
PS the references at the back of Qblade's buide book PDF are useful, but most will be difficult to obtain. See if you can find the last one:
TANGLER, J.: The Nebulous Art of usingWind-Tunnel Airfoil Data for Predicting Rotor Performance, NREL/CP-500-31243, National Renewable Energy Laboratory, Golden, Colorado, 2002Which is probably available on the NREL website somewhere.
Apart from that I would only recommend looking for the book by Martin Hansen, Aerodynamics of Wind Turbines. Earthscan, London, 2nd Edition, 2008.
All the rest seem to be about small subjects of performance correction factors, not the "big picture".
I've got a tonne more if that's not enough for you!