It's tough to say where English Prose Style belongs on the writer's bookshelf. It's more of an artifact than a practical guide to writing well. It was written for a British audience in the early 20th century, which means Read sometimes discusses how you might extend your sentences to 100 words or more by joining clauses with colons and semi-colons. This style of writing just isn't relevant anymore.
In many parts of the book, though, it's not so much an 'artifact' as a style guide based on historical principles, and in this way "English Prose Style" is fascinating to read. If you reorganized it a bit, you could turn it into a book on the history of English prose style.
I wouldn't recommend this to someone who wants to learn how to write. Instead, browse The Elements of Style or commit yourself to reading a more complete treatment of modern writing like The Oxford Guide to Writing. As a book about writing, though, English Prose Style is rich. It's a diptych that splits English into the usual "Composition" and "Rhetoric," but written for the person who loves to break down beautiful styles. A reviewer for the New York Times wrote that "'English Prose Style' appealed to the vivisectionist in me, the part that enjoys dismantling butterflies and trying to cement them together again." And Read is punctilious about proper style: he doesn't hesitate to accuse Santayana of not being able to hold a rhythm or Swift of occasionally writing "like a servant girl."
The book's practical strengths are its use of extensive use of excerpts and its historical lessons.
Read will reproduce several pages of prose in order to give the reader a full sense of different styles, and the book really benefits from this, because any comment on style that limits itself to a single sentence or a single paragraph is incomplete. You need a whole set piece or a whole essay to really understand the intention behind the sentences and phrases and words. And a natural consequence of this is that the book is filled with great writing. It's hard to come away from it without being inspired.
Another consequence is that there is no room for extended discussions. The book is often short on details and long on (enlightening) theoretical discussions. But not always consistently. Occasionally, you run into an exposition of specific rules, like types of sentences and ways to end a passage. These usually feel unnecessary, or leave you wondering why he doesn't also elaborate on specific techniques of narrative or fantasy. Ultimately, though, I'm happy he left the details of composition out of most chapters: Read is strongest in his theoretical passages.
On the subject of practical learning material, if you're careful about what you pick up and what you leave behind, you can take a lot from English Prose Style. Read's specific advice is out of date, and his philosophical comments are sometimes unusual ("The sentence as a unit in prose style is best approached from the evolutionary standpoint suggested by Jespersen. ...Jespersen says... that we must think of primitive language 'as consisting (chiefly at least) of very long words, full of difficult sounds, and sung rather than spoke'." Is this helping?), but always thoughtful and thought-provoking. But Read's book is historically grounded, an attribute that's missing in most of the how-to books I've looked through. He offers insight into the prose elements that were valued by Jonathon Swift, Robert Southey, and other old-time greats. You'll learn what historically defined prose and its rhetorical elements, which makes certain sections of the book timeless.
I pick up Read's book when I want to meditate on writing, to learn about prose's roots, to get a little inspiration, to chew on one of his philosophical points, to watch him pick apart esteemed writers. I recommend it.