Why is marking up a book indispensable to reading? First, it keeps you awake. (And I don't mean merely conscious; I mean awake.) In the second place, reading, if it is active, is thinking, and thinking tends to express itself in words, spoken or written. The marked book is usually the thought-through book. Finally, writing helps you remember the thoughts you had, or the thoughts the author expressed.
Co-founder of the Great Books Foundation and chair of the Board of Editors of Encyclopædia Britannica, Mortimer Adler (1902-2001) explains how to become an active reader. Consider what "owning" a book means to Adler, and why he characterizes reading as a "conversation between you and the author."