What a wonderful resource.
There are many of these books out for not only World War I, but also the second World War, Korea, Vietnam, even during peace time. If you know the Battalion or Regiment of your ancestor, search for the "yearbook," and you may even find their photo. My uncle has my Grandfather's WWII yearbook from 1942. I'm trying to get him to lend it to me so that I can scan it as this one is done. Inside are the photos of the nearly 3,000 soldiers of the Division.