"This book is more interesting."
Translation:Bu kitap daha ilginç.
'kitabı' either is third person singular possessive or specific direct object object (accusative case).
onun kitabı = his book
kitabı okuyorum = I am reading the book ('I' = nominative, 'the book' = accusative)
In the above mentioned sentence 'book' is in nominative case. It is the subject of the sentence.