to each two-book volume, preferring instead the use of book titles: e.g. The Lord of the Rings: Vol. 1, The Ring Sets Out and The Ring Goes South; Vol. 2, The Treason of Isengard and The Ring Goes East; Vol. 3, The War of the Ring and The End of the Third Age. However, these individual book titles were dropped, and after pressure from his publishers, Tolkien suggested the volume titles: Vol. 1, The Shadow Grows; Vol. 2, The Ring in the Shadow; Vol. 3, The War of the Ring or The Return of the King. Because the three-volume binding was so widely distributed, the work is often referred to as the Lord of the Rings "trilogy". In a letter to the poet W. H. Auden, who famously reviewed the final volume in 1956, Tolkien himself made use of the term "trilogy" for the work though he did at other times consider this incorrect, as it was written and conceived as a single book. It is often called a novel; however, Tolkien objected to this term as he viewed it as a heroic romance. The books were published under a profit-sharing arrangement, whereby Tolkien would not receive an advance or royalties until the books had broken even, after which he would take a large share of the profits. It has ultimately become one of the best-selling novels ever written, with 50 million copies sold by 2003 and over 150 million copies sold by 2007. The work was published in the UK by Allen & Unwin until 1990, when the publisher and its assets were acquired by HarperCollins. Editions and revisions In the early 1960s Donald A. Wollheim, science fiction editor of the paperback publisher Ace Books, claimed that The Lord of the Rings was not protected in the United States under American copyright law because Houghton Mifflin, the US hardcover publisher, had neglected to copyright the work in the United States. Then, in 1965, Ace Books proceeded to publish an edition, unauthorized by Tolkien and without paying royalties to him. Tolkien took issue with this and quickly notified his fans of this objection. Grass-roots pressure from these fans became so great that Ace Books withdrew their edition and made a nominal payment to Tolkien. Authorized editions followed from Ballantine Books and Houghton Mifflin to tremendous commercial success. Tolkien undertook various textual revisions to produce a version of the book that