In a book section, the row with Contents, History and Save now gets its background at the moment the toolbar reaches it as you scroll, and the background fades in rather than appearing all at once. If your device is set to reduce motion, it appears without the fade.
The Books editor's bar fades in as the toolbar reaches it
In a book section, the bar at the top now fades in when the toolbar meets it, instead of appearing as soon as you scroll.