The verb “нравиться-понравиться”
Hi! We say Мне очень понравилась эта книга. We can't use the Accusative case in this sentence because in this construction the noun книга is the subject of the sentence, so we use the Nominative case. This construction is not the same as the sentence "I liked this book very much" in English because in English it is not a subject, but the direct object.