Although Fearful Symmetry was Frye's first book, it was not the one he was famous for; that would be The Anatomy of Criticism. It became the preeminent textbook for literary critics. Anatomy of Criticism was a difficult book: for specialists. He wrote another one, a simplification of 'Anatomy', called The Educated Imagination. For those not up for that there's a study guide, in essence a condensation of Frye's first condensation. You might start with that and work your way up -- like a scholar does.
Here are two significant things that Frye said about Fearful Symmetry.
At one point he said that if had it to do again, he would have written a simpler explanatory treatment of Blake, like Percival's Circle of Destiny.
After Fearful Symmetry was done, he said that it had within it all he had to say with The Anatomy of Criticism.
So it appears that Frye thought he had written, with FS, a particular case of his thesis, while 'Anatomy' was the general case. Since that time many Blakeans have attended to Blake's poetry, pictures and ideas exclusively, to the exclusion of other poets and artists. They specialize in Blake, like a doctor might specialize in Ophthamology and know little or nothing about other fields of medicine.
A major convention of Western literature is the way in which stories get told. In The Educated Imagination (p. 52) Frye suggests that each story represents episodes in the story of literature itself . As he views it, all literature tells a largely cyclical story--"the story of the loss and regaining of identity" (p. 55). It can be seen in the hero's quest, where the hero leaves the safety of his society to face a monster and returns, or in the lover's plight, where the man is attracted to a woman, and marries her and is buried by her. But it's most complete representation in the West is the Biblical story of the Fall of man from his original home and the eventual return to a promised land or a heavenly kingdom."
Writing Fearful Symmetry Frye had perceived that Blake's opus was a Circle of Destiny with a departure and a return. Early in Blake's poetic development his circle had two points, reflected in two Contraries, Urizen and Luvah. But it soon became Fourfold: Blake's Myth, the System that he Created (Erdman 153). It had as its biblical source the 1st Chapter of Ezekiel, with the 'four living creatures. If you read Ezekiel 1 you can't help seeing their resemblance to the Four Zoas, forming the structure of Blake's unfinished masterpiece called The Four Zoas.
When you form the habit of recognizing the fundamental quality of the images Blake presents, you will be able to see the same kinds of images in the works of other writers. And if you have some familiarity with the Bible you may recognize the original sources of ideas, values, images that appear throughout literature.