Complex sentence: Zelda's novel had drawn heavily on her own life, as had Scott's previous writings; but Scott was irked because the novel he'd been working on for four years drew on many of the same events in their shared life.
Split into simpler sentences: Zelda's novel had drawn heavily on her own life, as had Scott's previous writings. But Scott was irked because the novel he'd been working on for four years drew on many of the same events in their shared life.