Sir Walter Newman Flower (8 July 1879 – 12 March 1964) was an English publisher and author. He transformed the fortunes of the publishing house Cassell & Co, and later became its proprietor. As an author, he published studies of the composers George Frideric Handel, Franz Schubert and (as co-author) Arthur Sullivan. He also edited the million-word journals of Arnold Bennett for publication.
Flower was born at Fontmell Magna, Dorset, England. He was the eldest son of John Walter Flower. After schooling at the Whitgift School he entered the publishing trade in London at the age of 17.
Flower trained under Lord Northcliffe at the Harmsworth Press, after which he joined Cassell & Co in 1906. Cassell was at that time in the doldrums, but Flower built up a stable of magazine titles that grew to dominate the British magazine market for many years. In 1912 he was given charge of the book publishing branch of the company, where he brought in such authors as Arnold Bennett, G. K. Chesterton and H. G. Wells.
In 1926, Cassell's magazines were sold to the Amalgamated Press, and Flower raised enough money to buy the book-publishing branch of the company, becoming proprietor and managing director in 1927. In 1938, shortly after receiving a knighthood for services to literature, he retired, but returned during World War II to look after the literary affairs of the company while his successor, his son Desmond, was on active service. Flower commissioned Winston Churchill's A History of the English-Speaking Peoples, which was eventually completed and published during the 1950s. During the war years, Churchill promised Flower that Cassell would be offered anything he later wrote about the war. The Times described the result, Churchill's The Second World War, as "perhaps the greatest coup of twentieth century publishing."