Complex sentence: Burroughs began writing the Barsoom books in the second half of 1911, and produced one volume a year between 1911 and 1914; seven more were produced between 1921 and 1941.
Split into simpler sentences: Burroughs began writing the Barsoom books in the second half of 1911, and produced one volume a year between 1911 and 1914. Seven more were produced between 1921 and 1941.