Anyone who wishes to `` release '' books , whether leaving it in a public place or passing it on to a friend , must register on the BookCrossing.com website , although there is the option to remain hidden when `` catching '' the find of a book . Anyone who wishes to officially participate in `` releasing '' books , whether leaving it in a public place or passing it on to a friend , must register on the BookCrossing.com website , although there is the option to remain anonymous when `` catching '' or recording the find of a book .