Osprey Publishing, which tends to be more 'accessible' and image-focused tends to have volumes upon volumes dedicated to Uniformology. They have multiple volumes for the various Armed Forces of World War II.

A few of their titles that may interest you:

"The Romanian Army in World War II" by Mark Axworthy

"The Royal Hungarian Army of World War II" by Dr. Nigel Thomas

"The German Army 1939-1945, in three volumes" Author unavailable

Another book, outside of Osprey publishing, that lends some pages to the French, Free French, Colonial Troops, low countries, etc:

"Army Uniforms of World War 2" by Andrew Mollo and Malcom MacGregor.