These two publishers have turned out several useful reference titles.
The Europa ones are better for the colour photographs and go a bit more into detail on the bits of kit. I`ve got three, WW1, WW2, and Vietnam Infantry in colour photographs, this series of books are probably some of the best available for providing themed airsofters with ideas for an impression. The WW2 book covers most of the nationalities involved, including some slightly unusual ones. There are also two books solely about Brit and U.S. webbing.
http://www.amazon.co.uk/s/ref=nb_ss_w_h_/202-3955591-6574208?url=search-alias%3Daps&field-keywords=europa+militaria
The Osprey books have some good photos, mostly period black and white, but the centre pages have about eight colour plates along with explanatory notes describing them. These books tend to go into the history and training etc. of a unit or conflict although they do have a few books concentrating solely on webbing etc.. I have several of them and are forever re reading them for a titbit of information
http://www.ospreypublishing.com/
As some of these books can be expensive or out of print I`d advise people to check Ebay as they appear quite regularly and often go fairly cheaply there 
I've found amazons marketplace (ie the resellers) the cheapest place to get osprey books.
No longer involved in airsoft.