My husband reads a metric xload of military history. The text books are fine on any decent ereader - he uses an old kindle 2 ereader. Kindle because the books themselves are generally cheaper on Amazon (and they are often expensive books). Cost of the media is a big thing, when choosing a device, because you'll likely spend more on the media than on the device.
For books with large maps, color charts, etc. basically he says no ereader is very good. A physical book is better. Except for carrying around, of course.
I should add that he does read some books with maps, charts, necessary color, etc. on his ipad. But there you're talking $$$. You might look into a tablet for the map / chart reading, though. There are less expensive ones than ipad, and there are some good reader apps that allow zooming, etc.