Shelf Life?

I have a colleague who lives in Boston but works for 10 days a month in Ireland and we regularly swap books. Yesterday he gave me a hardback copy of Elmore Leonard's 'Up in Honey's Room', first published May 2007, that he'd picked up for 50 cent at a sale in his local library. It's in very good condition and is a long way from it's last legs. I know libraries have fixed space and they need to sell on old books to create space for new ones, but I'm wondering as to how they make decisions about which stock to push off out into the wider world? I would have thought that a recently published Elmore Leonard book would have merited more than an 18 month shelf life? Or is this the standard library shelf life of a novel these days, with only the most popular books having a protracted period of lending? Or would this have been sold on because the library has a policy of replacing popular fiction books every 18 months regardless of wear and tear? I'm just curious as to what a typical library shelf life is and how libraries make decisions about which books to sell on? Can anyone enlighten me?