The book is 'hers'. [=the book belongs to her; it is her book]
a former professor 'of hers' [=one of her former professors]
That face 'of hers' [=her face] is hard to forget.
(..) What makes you think you can trust him? What makes you think you can believe anything my brother told you? And in all the time you knew him... Did he ever mention my name? Did he ever mentioned hers? - Why should he... - Keep secrets. You tell me? - I trusted him. - That's a boy's answer. (..)2011 Harry Potter and the Deathly Hallows: Part 2
(..) If it's true what you say, that he has the Elder Wand, I'm afraid, ... you really don't stand a chance. Well, I suppose I'll have to kill him, before he finds me, then. You're sure that's hers? Positive. Well? How do I look? Hideous. You can give that to Hermione to hold. Alright, Griphook? (..)2011 Harry Potter and the Deathly Hallows: Part 2