From a 2006 post (by Bert Webb) that has stood the test of time.

A new — or new looking — book is a treasure.  In my experience, however, I have found that a well-marked book, becomes more like a treasured friend — one that you enjoy seeing again and again.  It becomes much more enjoyable than a sterile copy that comes straight from the bookstore.  Don’t be afraid to mark up the books that you love.