In singular "she - her" translates to "sie - ihre", meaning only one female. There is also the formal "you - your" which means "Sie - Ihre". This could mean singular or plural, but is always formal. In plural there is also "you - your" meaning "ihr - euer". So the difference between Ihre and eure is the degree of fomality.

Yes, all three. Her books. Their books. Your books (formal address).