The US War Department circulated weekly lecture/discussion booklets on topics of interest to GIs, ranging from the abstract, such as US foreign policy, to the practical, such as living in the field. The scans are of varying quality, and the un-proofread OCR'd versions (text, epub, etc.) range from worthless to mind-destroying, but if you're looking for a view of countries and regions the world from a soldier's perspective, there is nothing better. The first link goes to a general search, followed by selected items of specific interest to us.
If you are writing anything to do with the characters' pre-HH experiences, such as how they got to Stalag 13, the 8th Air Force issues are essential, especially "On Base" and "Day and Night". "Homefront USA" has an excellent section on what a soldier isn't allowed to say in a letter home. In addition to the specific volumes linked below, the Internet Archive has quite a few more, all of them interesting. You can find them at the first link on the list.