The urbis site is a steal from an older group that - as far as I know- started the idea.
Take a look at http://www.bookcrossing.com
I thought I had posted this site about a year ago when I discovered it, but my search revealed nada. Sigh
I have been doing it awhile. The manager at my local Shaw's market has been really good about letting me place books among the shelves and plants. I have been lax about recording the books - ah, well, even I am not perfect !