Many questions about a book can only be answered by connecting passages chapters apart, such as a cough heard three times across a horse's life. We introduce CrossChapterQA, 1,853 such questions over 14 public-domain books. Each question lists the two to six chapters it needs (spanning half the book at the median) and the relation linking them, cites evidence from each, and has two independent reference answers graded blind. We use it to build LadderRAG, which indexes a book as passages, summaries and book-level cards for about $0.41 and answers a question for under a cent. Its first version failed mostly at retrieval. We redesigned it four times, steered by an LLM judge's verdicts and our analyses of failed answers: a design-level counterpart of RLAIF and RLHF in which feedback changes the architecture, not model weights. We also report the two redesigns that made answers worse. On the 191 questions its first version got wrong, the final version retrieves over twice as many required chapters on held-out books (recall 0.36 to 0.79), and ten annotators rate its answers 6.5/10 on average (764 ratings).

翻译：暂无翻译