Here's an article by Mortimer Adler, entitled How to Mark a Book.
Oh, my. It brought a rush of blood to my head to even think about this. But I found myself nodding along with his ideas! Agree or disagree, it's worth a read and a think! Here's a snippet:
Confusion about what it means to own a book leads people to a false reverence for paper, binding, and type - a respect for the physical thing - the craft of the printer rather than the genius of the author. They forget that it is possible for a man to acquire the idea, to possess the beauty, which a great book contains, without staking his claim by pasting his bookplate inside the cover. Having a fine library doesn't prove that its owner has a mind enriched by books; it proves nothing more than that he, his father, or his wife, was rich enough to buy them.
There are three kinds of book owners. The first has all the standard sets and best-sellers-unread, untouched. (This deluded individual owns wood pulp and ink, not books.) The second has a great many books-a few of them read through, most of them clipped into but all of them as clean and shiny as the day they were bought. (This person would probably like to make books his own, but is restrained by a false respect for their physical appearance.) The third has a few books or many-every one of them dog-eared and dilapidated, shaken and loosened by continual use, marked and scribbled in from front to back. (This man owns books.)