Active voice occurs when the subject is actively pursuing the object. Passive voice is when the object is happening to the subject. Sentence 2 is written in passive voice rather than active voice because it says the book was written by Christie. Active voice would say that Christie wrote the book
When the subject is actively doing something in the sentence, it is considered active voice. When the subject is being acted upon or receiving action, it is considered passive voice. The sentence about Agatha Christie's book is the sentence in passive voice. The book was written by Agatha Christie is passive voice also because it uses some form of the verb "to be" in the sentence as well.
Murder on the Orient Express was written by Agatha Christie. This sentence is in passive voice because the subject (Murder on the Orient Express) is receiving the action. To change this sentence into active voice, the subject needs to do the action; therefore, the active voice version of this sentence should say "Agatha Christie wrote Murder on the Orient Express." The subject (Agatha Christie) is doing the action (wrote) to the object (Murder on the Orient Express).