Complex sentence: After Mallory died, Cottie wrote a memoir of him, which was never published, but nonetheless provided much of the material used by later biographers such as David Pye and David Robertson and a novel "Everest Dream".
Split into simpler sentences: After Mallory died, Cottie wrote a memoir of him. Her memoir was never published, but it provided much of the material used by later biographers such as David Pye and David Robertson and a novel "Everest Dream".