The Fly-fisher's Entomology is the archetype fly-fishing how-to book.
Most fly-fishing historians credit Ronalds with setting a literature standard in 1836 that is still followed today.
 Describing methods, techniques and, most importantly, artificial flies, in a meaningful way for the angler and illustrating them in colour is a method of presentation that can be seen in most fly-fishing literature today.
As the name implies, this book is mostly about the aquatic insects—mayflies, caddisflies and stoneflies—that trout and grayling feed on and their counterpart artificial imitations.
 Less than half the book (chapters I–III) is devoted to observations of trout, their behaviour, and the methods and techniques used to catch them.
Most of this information, although enhanced by Ronalds' experiences and observations, was merely an enhancement of Charles Bowlker's Art of Angling (first published in 1774 but still in print in 1836).

Ronalds introduced several new ideas, however, in Chapter I.  His experiments and observations led him to describe and illustrate the trout's Window of vision, a concept an understanding of which is still essential today.
Vincent Marinaro, in his classic work In the Ring of the Rise (1976), credits Ronalds with discovering and documenting this window and includes a reproduction of plate II–Optical diagrams in his book.
 Ronalds’ physicist brother Sir Francis Ronalds quantified the phenomenon for Alfred using his knowledge of optics.
In the sub-chapter "Haunts", through discussion and illustration (plate I), Alfred Ronalds introduces the idea known today as reading the water to help the angler identify the most likely locations in the stream to find trout.

The real meat of Ronalds' book was Chapter IV: Of a Selection of Insects, and Their Imitations, Used in Fly Fishing.
 Here, for the first time, the author discussed specific artificial fly imitations by name, associated with the corresponding natural insect.
Organized by their month of appearance, Ronalds was the first author to begin the standardization of angler names for artificial flies.
 Prior to The Fly-fisher's Entomology, anglers had been given suggestions for artificial flies to be used on a particular river or at a particular time of the year, but those suggestions were never matched to specific natural insects the angler might encounter on the water.
 The following is a typical discussion:
No.
28.
Green Drake [Plate XIII].
This fly, proceeding from a water nympha, lives three or four days as shown; then the female changes to the Grey Drake (No.
29.),
and the male to the Black Drake (see p. 89.).
The Green Drake cannot be said to be in season quite three weeks on an average.
Its season depends greatly upon the state of the weather; and it will be found earlier upon the slowly running parts of the stream (such as mill dams) than on the rapid places.

Imitation
Body.
The middle part is of pale straw coloured floss silk, ribbed with silver twist.
The extremities are of a brown peacock's herl, tied with light brown silk thread.

Tail.
Three rabbit's whiskers.

Wings and Legs.
Made buzz from a mottled feather of the mallard, stained olive. (
See Dyes, Chap.
II.
p. 35.
article 4.)
To make it with wings in their state of rest, part of a feather similarly stained must be used, and a pale brown Bittern's hackle, or in case of need, a partridge feather must be wrapped round the same body under the wings.

— The Fly-fisher’s Entomology, 1849
The Fly-fisher's Entomology is the archetype fly-fishing how-to book. Most fly-fishing historians credit Ronalds with setting a literature standard in 1836 that is still followed today.  Describing methods, techniques and, most importantly, artificial flies, in a meaningful way for the angler and illustrating them in colour is a method of presentation that can be seen in most fly-fishing literature today. As the name implies, this book is mostly about the aquatic insects—mayflies, caddisflies and stoneflies—that trout and grayling feed on and their counterpart artificial imitations.  Less than half the book (chapters I–III) is devoted to observations of trout, their behaviour, and the methods and techniques used to catch them. Most of this information, although enhanced by Ronalds' experiences and observations, was merely an enhancement of Charles Bowlker's Art of Angling (first published in 1774 but still in print in 1836).
Ronalds introduced several new ideas, however, in Chapter I.  His experiments and observations led him to describe and illustrate the trout's Window of vision, a concept an understanding of which is still essential today. Vincent Marinaro, in his classic work In the Ring of the Rise (1976), credits Ronalds with discovering and documenting this window and includes a reproduction of plate II–Optical diagrams in his book.  Ronalds’ physicist brother Sir Francis Ronalds quantified the phenomenon for Alfred using his knowledge of optics. In the sub-chapter "Haunts", through discussion and illustration (plate I), Alfred Ronalds introduces the idea known today as reading the water to help the angler identify the most likely locations in the stream to find trout.
The real meat of Ronalds' book was Chapter IV: Of a Selection of Insects, and Their Imitations, Used in Fly Fishing.  Here, for the first time, the author discussed specific artificial fly imitations by name, associated with the corresponding natural insect. Organized by their month of appearance, Ronalds was the first author to begin the standardization of angler names for artificial flies.  Prior to The Fly-fisher's Entomology, anglers had been given suggestions for artificial flies to be used on a particular river or at a particular time of the year, but those suggestions were never matched to specific natural insects the angler might encounter on the water.  The following is a typical discussion:
No. 28. Green Drake [Plate XIII]. This fly, proceeding from a water nympha, lives three or four days as shown; then the female changes to the Grey Drake (No. 29.), and the male to the Black Drake (see p. 89.). The Green Drake cannot be said to be in season quite three weeks on an average. Its season depends greatly upon the state of the weather; and it will be found earlier upon the slowly running parts of the stream (such as mill dams) than on the rapid places.
Imitation
Body. The middle part is of pale straw coloured floss silk, ribbed with silver twist. The extremities are of a brown peacock's herl, tied with light brown silk thread.
Tail. Three rabbit's whiskers.
Wings and Legs. Made buzz from a mottled feather of the mallard, stained olive. (See Dyes, Chap. II. p. 35. article 4.) To make it with wings in their state of rest, part of a feather similarly stained must be used, and a pale brown Bittern's hackle, or in case of need, a partridge feather must be wrapped round the same body under the wings.
— The Fly-fisher’s Entomology, 1849
The Fly-fisher's Entomology