Time for the handcuffs?
I wouldn't lend someone's book on to anyone else, but I must admit I'm terrible for remembering to return them. I actually don't borrow books anymore for this reason (so there's the flip side ). I only borrow from my mum as I see her regularly and if it comes to it she can just reclaim it from my house herself.
I'm quite happy to lend books out though and take a fairly laid-back approach to if/when they are returned. most people I've found are far more responsible about these thigns than i am.