hey! it would be sus libros because
- nuestros libros (masc.) would mean “our books”
- nuestras libros (fem.) would also mean “our books”
- sus libros would mean “his/her/their books”
- tus libros (informal) would mean “your books”
i’d suggest getting SpanishDict on the app store if you’d like to know more :) hope this helps x