Complex sentence: Burroughs, as he had done in prior Barsoom novels, casting himself as John Carter's nephew, entrusted with Carter's manuscript of another Martian tale, actually mentions these games with Shea in the opening pages of the novel.
Split into simpler sentences: Burroughs, as he had done in prior Barsoom novels, cast himself as John Carter's nephew, entrusted with Carter's manuscript of another Martian tale. He actually mentions these games with Shea in the opening pages of the novel.