Most of the "name brand" travel guides are for fly-by tourists (though I do appreciate Frommer's guides much more than the rest of the big names). Well, if those books are for tourists, then Moon's handbooks (along with Lonely Planet's guides) are for TRAVELERS. And Moon's Wyoming Handbook is, as others here have said, one of their best. It's thick, it's juicy, it's meaty, it's expansive, it's authoritative and wry. So wherever you are in that great big "empty" terrain, it's got some practical information for and historical and cultural insight into places all around.
Wyoming has fewer people than any other state (yes, fewer than Rhode Island and Alaska). But it's places of interest are many and varied, though scattered far and wide. You need a good guide and a GOOD READ to cover the miles and the days. I admire author Don Pitcher's efforts here.
If you choose one guidebook, make it Moon's Wyoming Handbook. If you'd like to get a second general guide to the region for comparison and cross-reference (including more descriptive listings of selected accommodations), I'd add Frommer's guide to Wyoming, which includes Montana as well.