For the first year or two after my visit to France, I continued my old studies, with the addition of some new ones. When I returned, my father was just finishing for the press his Elements of Political Economy, and he made me perform an exercise on the manuscript, which Mr Bentham practised on all his own writings, making what he called 'marginal contents'; a short abstract of every paragraph, to enable the writer more easily to judge of, and improve, the order of the ideas, and the general character of the exposition.

~John Stuart Mill, Autobiography chapter III, “Last Stage of Education, and First of Self-Education”

I love the method outlined here and think it works equally well for the reader as for the writer, especially when applied to difficult books. Mill was almost exactly half my age when he returned from France and began this work (in other words, he was just over 15—and by this point he had, famously, already undergone homeschooling of the intensest variety).