Yeah, but if you're referring to a book, it's customary to refer to it by its main title, rather than just its subtitle. Especially when the subtitle is also the name of a different book, even if that book is a fictional book within the book.

If I started talking about how great The Modern Prometheus is, people would generally either a) be very confused, or b) think I was a totally pretentious dickwad for not simply calling it Frankenstein.