They don't get read past the first chapter.
I doubt there is any hard proof for showing this scientifically -- no way of knowing what happens to a book once it's on your bookshelf (though maybe your digital book could be tracked). This is believable, though, with the way I see people gather and never use information.
I see athletes buy 12-wek or 30-day programs and never complete the entire program. Know people who buy audio programs and have never inserted the second CD. People pay for memberships, sign a 12-month contract, and never show up to use what they paid for.
So to put yourself in the top 10%: Complete everything you start. Yes, it's that simple.