There are 3 books I depend on for sourcing and naming of the wildflowers that I find in Rocky Mountain National Park.
“Wildflowers of Colorado, Field Guide” by Don Mammoser with Stan Tekiela (2007) — I always start here. It has excellent pictures and descriptions. I find 85% of the wildflowers in this book. It’s also small enough to carry along on a hike.
“Southern Rocky Mountain Wildflowers” by Leigh Robinson (1999) — a good second stop if the first book fails me. Often the photos and descriptions are just different enough to help me identify a wildflower that didn’t seem to be included in the other book.
“Rocky Mountain Flora, Second Edition” by James Ells (2011) — if the first two books don’t have it, this one will. Includes shrubs, fungus and other plants that are not technically considered flowers.