Puzzle #153: Book Arrangements

You have five reference books. Two of the books are a two-volume set. How many orders can you arrange the five books in if the two books in the two-volume set must be next to each other, but the order of the two volumes does not matter?

Submit your answer to Gene Wirchenko <genew@telus.net>. Your answer should be in the form of a proof. That means to show how your answer must be correct. The deadline is Wednesday, May 11, 2016 at noon Pacific Time. I will post the answer shortly after.