A chapter repeats parts of the previous chapter. Why does this happen and is it fixed?

Each chapter is written in its own AI request, so the model only knows about earlier chapters through the continuity notes BookyAI sends along. As of version 1.9.1 those notes are much stronger: every chapter is written with an explicit rule to begin where the previous one ended and never re-narrate what the reader already has, each chapter's notes record exactly where it ends, and — new for non-fiction — a ledger of what every chapter covered is carried forward, so later chapters build on earlier ones instead of re-explaining them. On top of that, BookyAI now compares every freshly written chapter with the previous one and, if it repeats passages, writes it again automatically. If you still see a repeated passage in an older book, regenerate that chapter: the new rules and the check apply to regeneration as well.