Ulysses was fantastic when I last used it to do a multi-chapter document. I separated the chapters into their own files, it remembered each chapter's scroll position so it was easy to look things up in each, and the default templates looked superb.

I stopped using it when it went to a subscription, though. I can't accuse a company that lets you write in Markdown, basically plain text, as rent-seeking, but I dislike my workflow being held hostage like that. Haven't really found a suitable, multi-document replacement yet.