Free books?

At the bus stop this morning on my way to work, I noticed this novel on the bench, covered by plastic, with the label "free" on it. I know nothing of Lin Anderson or Dark Flight. I know now that she is a Scottish crime writer (and lo and behold, she has a blog too). My first reaction was: "A free book? Hmmmm... Somehow I don't think it's a good one." To be honest, I thought it looked like a suspicious gift. I never lend books (been burned too often), let alone give them. Not books that are worth reading, as they are always worth rereading. I love owning books and I don't like to share them. I thought this one might be soaked in arsenic, or be infected with bubonic plague, or something of the sort. So I published the picture on my Facebook page. And a friend of mine (my wife's cousin) explained to me that "there is a whole movement of leaving free books you have enjoyed on the tube. It’s a pay it forward moment." And I feel stupid now. I find it a lovely gesture, like done by literary Good Samaritans. And, like my wife's cousin pointed out, the one who did this cared enough to cover it. Still, I don't give books worth reading.