Materialism
It is clear that what is most important to many characters in the novel is what money can buy them. The reader sees this not only with Gatsby, who throws his party so that Daisy one day will hopefully walk in and be impressed with his things, but with other characters too. The owl-eyed man in Gatsby's library is so impressed that the books are real! Tom buys a puppy for Myrtle and also feels a bit like he owns her. This obsession with 'things' was very prevalent in the 1920s all around.