Complex sentence: Sir Max Pemberton was the editor of boys'magazine Chums in its heyday, and Cassell's Magazine from 1896 to 1906, in which capacity he published the early works of Austin Freeman, Clifford Ashdown, William Le Queux and Bertram Fletcher Robinson.
Split into simpler sentences: Sir Max Pemberton was the editor of boys'magazine Chums during its heyday. Between 1896 and 1906, he also edited Cassell's Magazine, in which capacity he published the early works of Austin Freeman, Clifford Ashdown, William Le Queux and Bertram Fletcher Robinson.