I know Swiss Cottage Leisure centre has a book swap that you can leave it at.
I've also heard of this app called Bookey that is a mobile app for book swaps! You just scan your books in and then start swiping on books that other people have uploaded. It then matches you with others if they like your book and you like one of theirs and allows you to message and arrange a meet-up. I tried it out yesterday and managed to swap one of my books and meet someone new which was interesting.