Use the Ngram viewer (click on this picture to take you to it) to graph trends of word usage over time, taken from Google's digitised book collection (that's more than 500 billion words!). Compare and contrast whatever subjects or phrases you are interested in. This graph shows how popular vampires have been in literature (funnily enough, even more so now - I wonder why?) and the growing increase in zombie mentions since their literary debut in the 1920s. Does this mean .... the Zombie Hoardes are coming?