for german ww2 overview book the encyclopidea of german tanks of world war 2 is excelent and highly recomended, covers prety much everything german with tracks or wheels including prototypes,rare veicules and captured. gun penetration charts , ammo comparaison , production figures etc. published by sterling publishing co isbn 1-85409-518-8 pbk or 1-85409-214-6 hbk
for the russians: russian tanks of world war 2 stalin's armored might is prety good , its a lot like the previous book but prety much only covers tracked armor, a lot of text in this one that goes through russian armor from the french ft-17 to the IS-3 explaining the development of each, also included are foreign veicules in russian service
now both of the previous books are overviews, it will give you a prety good idea of the role of each veicule but arn't realy suitable as reference books for modeling
for individual veicules i'm realy starting to like anything that Thomas L Jentz and Hillary Doyle write when they have a book on what i'm building, everything from them that i have seen so far has been top notch with a lot of research put into them, they are usualy a bit pricy though
i can't realy give too much information on allied armor or more modern stuff as i have almost nothing on those subjects (my main area of interest is the ww2 ostfront)