Tracy K. Smith grew up in a house lined with books, an eclectic library that included dime-store mysteries, 19th-century novels, science fiction paperbacks, Shakespeare’s sonnets and Reader’s Digest Abridged Classics. It seemed vast to her as a child, but soon she’ll have full run of the world’s largest library: On Wednesday, Smith was named the new poet laureate of the United States. “I was stunned,” she said from her office at Princeton University, where she is director of creative writing.