The novel, originally based on Twilight fan fiction, was released in 2011, so people have had plenty of time to pick up their own copies, or even to watch the movies based on the books if you aren’t much of a reader.
Maybe the assumption was made that if you’re interested in attending an event about a movie where people read the book, you’re already familiar with the book in question.
Since Book Club is all about a group of women who read the book, and how it affects their lives, it makes sense that copies of it would be used as decorations.