I never ever lend books. I know it makes me seem like such a pain some times but honestly, I can't do it.
Partially because I frequently go to my books and manga for inspiration, skimming through them for something that will inspire me, or make me burn with a new idea or feeling. With that, it means I can't ever have one book missing from my shelf, most certainly those that are part of a series.
Also, I don't know what kind of reader I would be lending to; I don't like doggie-earing readers, or spine-breakers, or even finger-lickers! It breaks my heart seeing books, especially if they were my own, being treated like that.