This is the best book on Fly Fishing I've purchased in a while. Definately *****. Originally designed to replaced their famous Western Hatches book, they cut it short when they had just finished the Mayfly section as the book was already pretty big. I hope they release versions for caddis and stoneflies soon.
The book is broken into the following sections:
1. Matching the Mayflies
2. Mayfly swimmers
3. Mayfly crawlers
4. Mayfly clingers
5. Mayfly Burrowers
The first section is devoted to how to collect mayflies, identify them and then match them.
As they discuss each mayfly they identify the importance of the hatch based on the distribution, emergence type, emergence time, Emergence duration and the Behaviour/Availability of the various insect stages. They also give a rough hatch chart identifying the time of emergence. The hatch chart is is just a rough guide as things vary wildly as you travel around the country.
They also identify patterns and how to tie them to match the various mayflies in their different stages. A lot of these patterns are very good, but it made me wonder why certain well known patterns were left out (John Barr emergers for example).
In the discussion of each species they also cover the behaviour of the bugs, which is helpful in knowing in what type of habitat you are likely to encounter them.
While this book I find excellent for the Colorado Rockies, someone in the Northwest would find it even more worthwhile, as both of the authors are from Oregon.