My 20-month-old son has been reading this with his father and me for about 6 months now, and it's one of his favorites. He usually asks us to read it at least once a day. It is a little long for him, so sometimes he'll get distracted by another toy half-way through, but he'll always come back to finish it.
There are lots of interesting things going on in the illustrations, which cover military history from the second world war up to the present. It's very neat to see how there is still something that interests my son, as his fascination shifts from dogs (shrieking "Puppy!" every time we got to K is for Kilo--detailing a K-9 unit) to Airplanes (Z is for Zulu--Zoom! as a jet takes off and my son makes zooming motions.) This book is a good choice for active boys as well as those who will sit still for longer periods.
I also really enjoyed that there is a more in-depth explanation of what each illustration depicts detailed in the back of the book. So, while on the page for L is for Lima the text reads "Marines, known as Leathernecks, make a beachhead landing," at the back of the book, you get more information telling you that the beachhead landing shown is from the Allied invasion of Normandy on D-Day. This makes the book interesting for me as a parent, and also adds to it's longevity as a part of my son's library.