Complex sentence: Saunders did not originally intend to write a novel (and had avoided doing so in the past), but the story of Lincoln cradling his son's body stayed with him, and he eventually decided to write about it.
Split into simpler sentences: Saunders did not originally intend to write a novel, and had avoided doing so in the past. But the story of Lincoln cradling his son's body stayed with him, and he eventually decided to write about it.