Say that the person has his/her/their own book or books.
Ella tiene ______ libros.
sus
su stands for: his, her, its, their IF "book" is singular
sus stands for: his, her, its, their IF "book" is plural, as it is here.
Sra
Ella tiene ______ libros.
su stands for: his, her, its, their IF "book" is singular
sus stands for: his, her, its, their IF "book" is plural, as it is here.
Sra