The strength of this book is its approach at SIMPLIFYING your selection of trout patterns. Hughes' basic contribution is to make the point that bugs of specific families (e.g. mayflies, caddisflies, etc.) can all be matched by the same fly style. All that needs to be changed to match the natural is color and size. For example, a parachute style fly can be used to match BWOs, PMDs, Hendricksons, and any other mayfly. What needs to vary is just the color and size.
The other helpful point from this book is the idea that basic pattern designs should be chosen to match water conditions, not specific hatches. For example, Catskill flies for rough water, Comparadun style for smooth water, Parachute style for water in between. This is in contrast to selecting pattern designs for specific bugs (e.g. comparadun for a BWO).
The true value of this book is the focused clarity on selecting and tying patterns for a simplified fly box. I think this book will be useful for beginners confused by the vast array of available patterns, and for more advanced anglers looking for clarity and simplicity in their pattern selections.