My mentors write in their books, usually in different colored inks. To me, they've made their books more valuable. When technology makes more intelligent advances, we should be able to reproduce their marks, notes, in different color inks. Last week I saw an annotated copy of Jane Eyre, and while there are many annotations, I like the idea of everyone having a chance to annotate. Imagine buying a classic and being able to pick the annotations you want and add them to your electronic edition. The pixels and electrons are yours forever. Here is a link to Brainpickings and an excerpt from How To Read A Book by Mortimer J. Adler and Charles Van Doren:
"When you buy a book, you establish a property right in it, just as
you do in clothes or furniture when you buy and pay for them. But the
act of purchase is actually only the prelude to possession in the case
of a book. Full ownership of a book only comes when you have made it a
part of yourself, and the best way to make yourself a part of it — which
comes to the same thing — is by writing in it.
Why is marking a book indispensable to reading it? First, it keeps
you awake — not merely conscious, but wide awake. Second, reading, if it
is active, is thinking, and thinking tends to express itself in words,
spoken or written. The person who says he knows what he thinks but
cannot express it usually does not know what he thinks. Third, writing
your reactions down helps you to remember the thoughts of the author."