At least for me! I was going through my refrences and library of art books frustrated that I felt like I have been getting nowhere in months. I found a file on my computer that was Vilppu's drawing manual. I have never understood an author more then Vilppu. His advice was perfect for how I felt. I read the entire book and picked up a few things here and there.
I love to study now because I used to be a slave to the model. I only copied the subject in front of me, I now analyze whats in front of me. With that idea in my head (thanks to vilppu) I have learned what studies can really do for you :)