There are two physical volumes of an old encyclopedia sitting on a shelf in a college library. One volume is labeled A-M and the other N-W. Is the third volume X-Z missing?
I believe this is the answer that makes the most sense.
It seems unlikely that there is a third volume X-Z because it would be so much shorter than the other two volumes. It would be a poor design choice.
The encyclopedia must not be using the English Alphabet. It seems that the Greek Alphabet is much more fitting. Alpha - Mu, and Nu - Omega. The uppercase Greek letters look similar to the corresponding English letters. (Some stylizations of the uppercase omega look a W, and the lowercase omega definitely looks like a w.)