What proportion of my books do I read more than once?
Probably 1 to 2%. Heck, I have a increasing number of eBooks at the moment that I still haven't even read once.
I'm not sure it matters though. That is simply a restriction I would not be willing to agree to. If I WANT to read my books 153 times I blankety-blank will and I wouldn't buy a book that I couldn't.