When it was first published in 1950 it was described as being far ahead of it's time.
In chapter three Marinaro describes what the trout sees, it's window of vision and the importance of the 'footprint' - the impression that the fly makes on the surface of the water and seen by the fish in the mirror before it reaches the window .
A Large Dark Olive Dun (Baetis Rhodani)
In my opinion the footprint is not always that important when fishing our northern freestone rivers. For example, when fishing the dry in riffles, this water tends to be faster, more turbulent and shallower - trout in this type of water are closer to the surface therefore their window is smaller and they will not get time for a good inspection of the faster moving fly, and they are usually not so fussy about what you chuck at them.
As the riffle slows and deepens into the pool then this is where we should, on occasions, think of the impression our fly is making in or on the surface. Here the fish may be lying deeper, it's window is larger and it gets a good view of the mirror around that window, and anything that is piercing or creating a footprint on that mirror.
So when I'm tying my dries, I try to cover whatever the fish may be on - emergers, drowned or crippled duns and a variety of duns in the hope that on any given day one of them will create 'The right footprint'