The guidebooks mentioned by state are excellent. In addition to tips on specific rivers, you’ll get maps with access sites.
To whet your appetite, here’s a site with links to some rivers by state, with some info on each:
Welcome to the Flyfishingconnection
If you’re thinking of going to Yellowstone this book by Craig Mathews is very good:
Amazon.com: The Yellowstone Fly-Fishing Guide: Craig Mathews, Clayton Molinero: Books
There are a variety of different water types—spring creeks (small flies delicate presentations), small fast freestone rivers (big bushy dries), slower big food-rich freestone rivers with a variety of hatches from small (tricos) to huge (salmonflies), and tailwaters (small flies and midges). Hatches will vary by latitude, water type (spring, small fast freestone, slow big freestone, tailwater, lakes) and water source (snow melt or other) same as in the East, but the big difference is the effect of altitude for us eastern guys, with hatches occurring at different times, early lower and later as you move up into colder water on the same stream.
Here are some very general “western hatch charts” that by their nature is very general, but it’ll give you some idea of the sequence of hatches if not the exact timing for specific waters. Main differences form our eastern waters will be PEDs, PMD’s, Callibaetis, ginormous stoneflies like the Salmonfly, Western Green Drake/Grandis and Flav (smaller and greener than the eastern green drake and hatches last a longer time, an important hatch in many places out west), hoppers in August are a bigger deal and a lot more emphasis on stimulator and caddis patterns (dries and emergers and specific western hatches like the Black Dancer in late summer and October Caddis in PNW in early fall that we don't have and cased caddis patterns like the brassy) and more emphasis on fast water patterns like humpies and trudes for steep gradient streams.
Lot’s of stuff like BWO’s, and other stuff you probably have for eastern stuff are good out west of course. Here’s a link if you want to get a head start on some tying:
Western Fly Hatch Charts
Do a lot of research and try to hit specific waters that you’ve mapped out in advance Google hatch charts on the waters you plan to fish to get background info and zero in on fly shops for local up to the minute advice when you get there.
Sounds like a great trip, you should have a blast!