"Taking turns" works for us. I also renew books very liberally, keeping them to the due date. We also go to the library very often, three, four times a month, having some books to read at the library and some we'll take home, using the share and take turns language with either makes it easier.
Of course, you know how some toys/books that are favorites, nobody wants to share those? With one particular book we had to "blame" the library and say it's theirs and they want it back and didn't want to share anymore. It was the only answer they'd accept.