BookCrossing are running an article about a book called The Revolution Will Not Be Televised in which it says that this is the first book to mention BookCrossing - but it's not! Hello World is! See p131:
If I combine lots of letters and make lots of words then collect them all together – as in this book for example – the book can be published and several people including me will, hopefully, earn money from it. If you buy a copy, you can install it in your brain (read it) then give it to your friend, who can install it in her brain and then perhaps give it away to a charity shop where it will be resold to someone else, or leave it on a bench somewhere as part of the new fashion for releasing books into the wild (footnote) and the cycle will continue. If, two years later, something comes up which reminds you of my book, you might want to think about it some more, and that is free of charge. And if you want to scribble in the margins and change words or cross bits out, that’s ok too.