"Spanish American War" by Brassey (
http://www.amazon.com/SPANISH-AMERICAN-1898-Brasseys-History-Uniforms/dp/1857532724)
is an excellent resource for uniforms of all the services - American, Spanish and Filipino.
Information on the US Army and Volunteers is very detailed, with plenty of illustrations and photos; Marines and Navy, not so much. One glaring error I've found is a colored illustration of Marines at Cusco Wells - the artist has the colors of the cheverons reversed. His dipiction shows red striptes with a gold backing, which is OPPOSITE of the correct gold-on-red.
Osprey has several titles showing Marines of this era, showing Marines (and other services/countries) in Cuba, Philippines, and Peking (Boxer Rebellion, 1900).
Neiderlander - I was not aware of the other differences in the Marine/Army blue shirt. Thanks for the info!