How can I see how the relationships stand at a certain chapter?

From version 1.9.22, open the Codex from the book's page and choose Relations. Under the relationship map there is a slider with a play button. Drag the slider to a chapter, or press play, and the map shows the relationships as they stand at that chapter: who has met, who has fallen out, who knows which secret. The changes that happen in that chapter are listed under the slider. "As the book begins" shows the starting point. This is also exactly what the AI is given when it writes that chapter.