I've been aware of this for a long time but never got round to joining in but this week I released 6 books in the wild around Trowbridge.
The idea is that when (or if) the book is found the recipient logs on to the bookcrossing site and registers that they have found the book and when they have finished reading it, it is released again in the wild, and so on ...
I'm not sure that I will do this again. I like the idea of sharing books in this way and although I have never found one I know I'd be excited if I did. However, there is no guarantee that the book will be found (or you may not know it is found as the finder doesn't register it) and a big part of my brain is screaming that this could be seen as a form of littering or as a suspicious package and I felt uncomfortable about leaving the books.
Still I've done it for now ( although I may go back in a couple of weeks and remove the books if they are still there - they're very visible and if they haven't been collected in 2 weeks then I don't think they will be) and it is another thing crossed of the list - 36 more to do.