His first posts in journalism included a period in Nottingham where he first met Arthur Mee, who was to become his lifelong friend. Then, in 1905, he joined Alfred Harmsworth's Amalgamated Press. There he again collaborated with Mee in producing the Harmsworth Self-Educator.
He contributed to the first edition of Mee's Children's Encyclopædia, which appeared in a fortnightly series from 1908 till 1910 before being published in eight large volumes. His contribution consisted of compiling articles on 'Famous Books' and 'Poetry'.
His greatest achievement was Harmsworth's Universal Encyclopædia, which – like the Children's Encyclopædia – was published first as a fortnightly series in 1920–22 and sold twelve million copies throughout the English-speaking world.
From 1914 to 1919, Hammerton was joint editor with Herbert Wrigley Wilson of the periodical The Great War:The Standard History of the All-Europe Conflict, published by the Amalgamated Press. The first volume was largely concerned with justifying Britain's entry into the war, and with encouraging the British people to sign up and fight. In its entirety, it ran to 13 volumes.
In 1933, his A Popular History of the Great War (in 6 volumes) was published. In his introduction to volume 1, he concedes 'Although it remains a storehouse of information for future students of the period, "The Great War," as that set of thirteen massive volumes was called, would now require to be largely re-written in light of later knowledge' - and so the Popular History was written "embodying the gist of post-war revelations and official documents". Each of the volumes covers a specific period (Volume 1: "The First Phase: 1914"; Volume 2: "Extension of the Struggle: 1915"; Volume 3: "The Allies at Bay: 1916"; Volume 4: "A Year of Attrition: 1917"; Volume 5: "The Year of Victory: 1918"; Volume 6: "The Armistice and After"). The complete work comprises 3,840 pages of text, along with over 100 maps and diagrams and some 800 photographic illustrations.
Subsequently he edited numerous literary works. Among the publications in his own name, one can mention a biography of J.M. Barrie and studies of Charles Dickens and Robert Louis Stevenson. He also wrote an autobiography, Books and Myself (1944).
After the death of Arthur Mee in 1943, he wrote in 1946 a biography of the latter entitled Child of Wonder.
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