The Elements of Style.

The Elements of Style is one of the most influential handbooks in English prose: small in size, direct in tone, and enduring because it keeps returning writing to its central tests of clarity, precision, and emphasis. William Strunk Jr. wrote it as a practical guide for students, but its brevity and firmness have given it a much wider life among readers, teachers, editors, and writers.

Why read The Elements of Style today?

Even readers who disagree with parts of its doctrine can still feel the force of its method. Strunk’s rules are short, memorable, and designed to sharpen attention. The book is useful not because it replaces judgment, but because it pushes prose away from vagueness, clutter, and self-indulgence. It remains especially helpful when you want to revise sentences down to their essentials.

What kind of classic is it?

This is a practical nonfiction classic: less a literary artifact than a durable working tool. It belongs on the shelf of anyone interested in writing, editing, rhetoric, or the teaching of English, and it is best read not as dogma but as a concentrated statement of one influential vision of style.

Further context

Wikipedia: The Elements of Style.