Vending machines, Warwick University Library
These ones are fairly simple, really. I left them in or near the exit slots of some vending machines with a note on saying "Free book". However, it is here that you will begin to notice the severe deterioration of my camera skills when people are nearby. I get panicked, and pictures come out badly. Apologies. What's nice to note is that when I returned to this area an hour later, all three books were gone. Success!