https://www.cnet.com/news/george-r-r-martin-game-of-thrones-struggling-winds-of-winter-book/

The first volume of Fire & Blood, the 989-page Targaryen family history he's about to publish, wasn't as complex, Martin said. 

"Fire and Blood by contrast was very simple," he said. "Not that it's easy, it still took me years to put together, but it is easier."