Sunny thinks that 'Boston' sounds familiar. Perhaps she heard it mentioned somewhere before. Maybe Magnu told her that's where he is from. Maybe in a film adaptation of her life that is the city she is also from, and she heard it mentioned there. Maybe it's a famous place and she heard it mentioned as a result of that. Maybe none and/or all of the above.

"Book?" She asks, which here means, "A field guide is a type of book, right? My brother likes to read books to find out information that is helpful."