"저는 책이 아닙니다."
Translation:I am not a book.
In this sentence, 저 (jeo) is the topic and it ends in a vowel so 는 (neun) marks the end of it. 책 (chaeg) is the subject and it ends in a consonant so 이 (i) marks the end of it. If the topic ended in a consonant, you would use 은 (eun) and if the subject ended in a vowel, you would use 가 (ga).
In English if the book were the subject then we would say "The book is not me.", but this verb "is" is like an equal sign which is negated here. So there really isn't an object. So, in Korean do they consider both nouns to be subjects and then one is marked as the topic? We call the second noun in this structure as a predicate nominative, recognizing that both are in the nominative case.