Undercover University by Frank Bell

Not a textbook of Japanese, though there are bits and pieces of Japanese-language material in it.  It's a first-hand account of imprisonment in a Japanese prisoner-of-war camp; Frank Bell was captured in the Far East (as we would say) in 1942 after a very short spell of active service with the Royal Artillery and spent over three years in various camps, where he and fellow officers tried to keep their minds busy by teaching one another things—often languages.  The book, produced by his widow after his death, contains a lot of reproductions of pages from notebooks from the camps, usually filled to the margin (paper was a luxury) with declensions of verbs and vocabulary lists.

The Pacific war doesn't loom as large in popular imagination as the European theatre of war does; perhaps any chance of comprehension was finally lost when the old Empire disappeared.  The Dunkirk evacuations and the Blitz still speak to us, but it's difficult for most of us now to imagine what a huge effect the fall of Singapore had on morale.  The geography is more difficult, too: trying to follow the American island-hopping campaign is very difficult unless you've got excellent knowledge of Pacific geography or you've got a map to hand.  This topic is probably one I'll keep going back to.  Anyway, I recommend the book if you can find it.