I've confessed to my fondness for maps more than once before here, I think ... both old ones and new ones. As a group, there's no doubt that they're the greatest form of recreational reading ever.
Montana maps are especially important to me, both because they're lots of fun to mentally wander through, and because they're practical guides to my ongoing roadtrips. They come in all sorts of forms -- I've got a small collection of old state highway maps; a set of big old maps of the National Forests; a few old railroad maps; some of Glacier topos; a whole bunch of odds and ends, and a couple of big paperbound atlases that I take with me on drives. (For that, I strongly recommend the volume that Benchmark Maps issued a year or so ago.)
The best close-up maps of the state, of course, are the 1:25,000 quads produced by the USGS, and a full collection of those would be a thing to have ... but both the money and the storage space would be a little much for me. You've been able to view them on the internet for a few years now, but they were hard to find and awkward to navigate. Recently, though, the Montana Historical Society made available a new website that has by far the best online map of Montana I've ever seen.
The site is at mtplacenames.org, and it's just made for browsing. When you start out, the data you see is that found on the official state highway map, but as you zoom in you begin to see more and more topographic data, and at the maximum zoom you're looking at scans of the USGS topo maps. The whole state is there, and it's a great resource ... and if you're like me you'll be able to spend endless hours wandering around.
(The site has another feature, too ... all of the text from the Historical Society's recent Montana Place Names book, each entry linked to the appropriate spot on the map. That's definitely fun, too ... but I have to confess the place names book was disappointing to me, and here the map itself is definitely the great attraction.)