Lately I have been in a researching mood, look for new techniques and artists. I have also been thinking about fundamentals, as you know a good base is the sinew of an endeavor.
Learn To Drawn The Human Anatomy From The Mind (nice catchy title) is a series of tutorial web videos that I have found interesting and useful. I love drawing straight from my mind even though past tutors have always gone on about having primary sources and secondary material.
This collection goes further than just going on about arbitrary construction lines (Like in a lot how to draw books I've read). It goes into his thought process, as well how you have to help you brain turn formula into function by looking at things like negative space.He also goes into looking at Da Vinci's artwork and relates it to his own theories and techniques. Heres the first video if your want to take a look.