There's no doubt (at least to me) that maps provide the greatest recreational reading imaginable. And old maps can be particularly fascinating, both for their period graphics and because they provide fascinating insights on how the human landscapes of the world are changing.
The Montana map that most people know, of course, is the official highway map that's been published by the state since 1935. The current version is computer-generated and informative and handsome, but for real visual appeal you just can't beat the state's earliest maps. The Department of Transportation website has scans of the cover panels of of those first maps, and all the pre-World War II ones are great. This one, I think, is my favorite: