There was a discussion over at the Miniatures Page about a good WWII painting resouce. I don’t normally do posts about commercial products, but you can consider this a public service review.
For my money, the best resource for WWII uniforms is the Armed Forces of WWII by Andrew Mollo.
I’ve painted a lot of WW2 figures on commission over the last several years, and I have found this book to be the one indispensable resouce. Virtually every uniform I’ve wanted has been in here. Where else would you find the uniforms and insignia of the WWII Hungarian Air Force. There are 365 color paintings in the volume, and many more black and white photos. There is also a lot of information about the organization and ranks of the various armies. All, in all, its highly recommended for WWII painters and gamers.