It all began with the “Twilight’’ series, which has the first of its two final movie installments, “The Twilight Saga: Breaking Dawn - Part 1,’’ hitting movie theaters this weekend. The book ignited a publishing industry trend that continues to see adults purchasing books written for teens. Young adult books are no longer for kids alone - and, as a result, sales are often out-pacing grown-up bestsellers, sometimes by millions.
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