More tanks.
Books like this are probably better for writers than artists though. I have a couple of big difficulties with drawing tanks (beyond all the normal difficulties of actually drawing). They are:
How big are people in relation to the tank (3d tank reference objects are brilliantly useful for what the vehicles look like, but useless at giving you a sense of scale) and what the hell is going on inside the tank. As it happens there are some pretty good videos of people exploring the inside of (some) tanks – some of the more well known vehicles have videos touring the crew components. But give me a well drawn diagram everytime.
Anyway, this is gonna take up a fair bit of the next six months… hope you like tanks!