The principle of
bookcrossing is to leave books you have read on the tube, bus etc. for others to pick up and read. But what you do beforehand is register your book on the website, stick a bookcrossing reference number in the cover (you can print off from the site), and then release the book into the wild. Hopefully whoever picks it up will of course read the book but more importantly go on the website, leave a comment and then let the book loose again.
Give it a go.