"That is his book."
Translation:वह उसकी किताब है।
I think there is a mistake in the english translation "her" instead of "his" because the opposite indian word is "उसकी" Not " उसका"
This sentence could be either 'That is his book' or 'That is her book'. उसकी/उसका/उसके conjugates with the object, not the subject. The object is किताब here, which is feminine, so it is उसकी regardless of whether वह is him or her.
As Zoobo7 said उसकी could be either his or hers equally in this sentance. किताब is feminine which determines the use of उसकी instead of उसका.