How do you say her books in spanish?

Answer 1

"Her books" directly translates to "Sus libros"

but "Sus" (plural) or "Su" (singular) can mean her, his, their, or your (formal).

To make clear its 'HER books', you can write 'sus libros de ella'

Another way to show possession is by saying "Los libros de la chica (or "la nina")." That sentence translates into "The girl's (or "the little girl's") books".

Answer 2

Ella devolvió el libro.