How do I tell BookyAI how my characters feel about each other?

From version 1.9.22, open the book's page, press Codex above the chapter list and choose Relations. Each row says what one character is to another, for example "Elisa is Tom's friend": pick the two people, then write or choose the kind of tie. Click the arrow at the end of the row to open the details. There you write how each of them feels about the other, because feelings are rarely equal ("in love, has not told him" and "sees her as a sister"), set the strength from faint to consuming, tick "Secret relationship" for an affair or a hidden alliance, and set "Begins in chapter" for people who only meet later. Press Save at the bottom of the Codex. From then on, every chapter is written with these relationships in mind.