"For native English speakers, the single greatest moral resource in the language is the nineteenth-century novel. I taught for a few semesters at a writing program and would always ask my students how many of them had read Middlemarch or Bleak House or Portrait of a Lady. It was a top-tier writing program, all highly competitive, excellent students, but a distressing number hadn't read some or even any of these books. Austen, Eliot, Dickens, Thackeray, Meredith, Trollope, James, Hardy, Conrad are, together with Balzac, Stendhal, Tolstoy, Dostoevsky, and Chekhov, a matchlessly deep and precious trove of wisdom.
Every language's poetic tradition is rich, but ours in English is very rich. Sixteenth- and seventeenth-century English verse is a source of many, many exquisite pleasures. You do have to work a little--the language is not colloquial twentieth-century English--but it's the root of our English. And then, the King James Version of the Bible. My favorite twentieth-century writer, D.H. Lawrence, wrote a lovely essay on growing up with the Bible called "Hymns in a Man's Life." The KJV Bible is even richer than Shakespeare, both psychologically and linguistically. (I don't mention Shakespeare only because I don't think your readers will need my recommendation.)
Well, that's a bare minimum. As for other writers besides novelists or poets, I would recommend Nietzsche, my favorite philosopher, as well as John Stuart Mill. Appreciating those two simultaneously is the challenge of a lifetime.
As for what not to read, I would say don't read your e-mail, or most of it. Don't read text messages, or tweets, or ads. Stay off Facebook. We all waste so much of our lives chatting, shopping, being assaulted by ads. And don't watch television. Television is an enormous realm, and there's a lot that's good--but it's very hard, almost impossible, not to find oneself relaxing into flabby, promiscuous spectatorship, just as it's very hard to eat only one or two potato chips."