I have a small book that measures 3 x 2 x 1/2 inches and fits perfectly in the pocket! It is from the COLLINS/Jane's Gem series. It is entitled "Tanks of World War II" and covers every major vehicle used in the War, including some notable variants. The publisher is Harper Collins copywrite 1995 and it's priced at 3.99 Net British pounds, $8.00 US , and $11.50 Canadian.
The COLLINS/Jane's Gem series also includes one of Modern Tanks (which I want to get), Modern Combat Aircraft, and Aircraft of World War II.
Good luck in finding it. It really is a great little book!