Impressive Books

I went to my attorney’s office the other day and was seated in the conference room. While I was sitting there, I was looking around the conference room at the impressive collection of law books.

I think they put you in there so you are impressed with all the knowledge your attorney has, so you don’t get too upset when you get that bill for 485 dollars an hour.
Though most of the law books contained dozens of volumes and were indexed with very specific descriptions, one set of law books consisted of just two volumes. The first book was A through M, (like Aardvark through Moron). The second was N through W, (like Ninny through Wombat).

At first, I wondered what happened to the last volume, X, Y and Z. Maybe it was being used to research a case, and was the only book missing from the shelf. But, then I figured out there was no missing volume. These two books were it. And the question is, why was there no X, Y, and Z?