While there are many poor instruction books on how to draw, there are a few excellent ones that were published in the 1920s through 1960s. One of my older instructors liked to joke that people write instruction books to pay for their groceries and those books have no real value. The books that I find extremely useful are all the books by Andrew Loomis (thank you, Illustration Academy friends for introducing me to this instructor) and George Bridgman. While Bridgman's books have been reprinted since their original publication, Loomis's books are extremely hard to find. By copying every single drawing from these books, I try to learn to think how the authors did in order to improve my drawing skills. Both of these instructors pay a lot of attention to the structure and 3-dimensionality of the figure, and all the parts of the figure. This type of exercise doesn't require a lot of time, concentration, or inspiration. All it needs is 30 minutes here, 30 minutes there. All the same, this kind of copying is very helpful and one can learn a lot about movement, structure, shape, form, muscles, proportions, bones, gesture and just about everything else.