Zbeckabee, Wikipedia also says in the table "100 million (usually published as three volumes)" but in the notes says "The Lord of the Rings is considered by most people to be a single book, because it was written and planned by the author to be such. Some people consider it to instead be a trilogy or series of three books, because it was originally published as a series of three volumes: The Fellowship of the Ring, The Two Towers, and The Return of the King. This move by the publishing house was due largely to post-war paper shortages as well as to minimize the price of the first volume to aid sales. In subsequent printings the book has sometimes appeared as a single volume, and in at least one case was split into seven. The figure of 300 million is an estimate of copies of the full story sold, whether published as one volume, three, or some other configuration." And another note that has me wonder is "This list was originally compiled in 1997, and has been updated using the sources shown, and others, including the articles on each title."
In the table, each Harry Potter book is counted individually. It still totally 397 million sold though.
I can't find another source (perhaps more reliable) for book lists though.