First appearing in the 1600s, when they were known as “manuals of principles,” drawing books quickly became an essential tool in the teaching of fine arts. But these books—which contained the work of renowned painters, printmakers, and sculptors—are given little attention. Why? Because they are scarce. In a rare retrospective that traces two centuries of teaching material, Museo del Prado celebrates its 200th anniversary with an exhibition of drawing books from its own holdings. —E.C.