Am reading an excellent little ebook on lighting portraits with natural light that is available for free from Rocky Nook press. It is a chapter from a book by Roberto Valenzeula. You can find it here.
(I stumbled on it when I went to the site looking for the ebook on transitioning from Aperture/Lightroom to Capture One, which is also free.)
I've also learned a few things from this little tutorial on light that was written for animators, but which uses photographic examples.