In general, I favour capital punishment for people who put marks in books. They'd be easy to catch, for almost all put their marks on only the first few pages. The effort of defacement seems itself to exhaust them, so that they seldom make it to the end of a long preface.
I've recently been immersed in a shallow vice: Star Wars novels. There are at least 275 of them, and I've maybe worked my way through twelve so far. It's a daunting task.
I had to laugh when in one of these tomes I encountered frequent use of a highlighter on some of the more profound thoughts. "Evil is as evil does" is but one example.
I know that truth is truth, no matter where it's found. But the poor chap who found inspiration in a Star Wars novel must really crack open a Chesterton essay or some C. S. Lewis; there is much deeper thought contained in their works - even in their fiction.