YouWARNING: This was the original answer, which I see now is wrong. The comments bellow the answer lead to that conclusion.
(You could restate the first as:
– Wem gehört dieses Buch?
– Ist es Ihr Buch, Frau Schmidt?
– Ja, es ist meines.
if you wouldn't mind to repeat the noun Buch. That's what genitive there does: it's avoiding repetition. Otherwise, you say it in nominative.
Other viewpoint would be that, since possessive pronouns already, well, imply possession, you don't need an additional genitive. You do the same in English, actually. Trying to state the second as you say, would be the same mistake as saying "That album is of hers".)