This recent Writer's Digest infographic includes this and many more literacy statistics—including the fact that the US is only the 7th most literate country in the world.
What can authors do to improve literacy? Well, the obvious answer is to write a book that everyone wants to read. How many people have you met who said they didn't read after they were done with school, but then they picked up Harry Potter and the Sorcerer's Stone, or Twilight, or The Da Vinci Code, and that got them reading again?
Another answer is to remove any judgement associated with reading "fun books," or "books for kids." I'll never forget a conversation I had with one of my husband's college friends. He told me he used to love to read, and we bonded over our mutual love of Patricia Wrede's Enchanted Forest Chronicles. Unfortunately, he graduated college with the idea that the only acceptable books were weighty, impressive books of the type he studied in classes. The result was that he'd read only a handful of books in the ten years since.
I find this is incredibly sad. Everyone should be able to read whatever they enjoy, whether that's the Harry Potter series, category romance novels, or the classics (there have to be people out there who actually enjoy the classics, right?). Reading has been proven to improve quality of life. Let's let people read whatever makes them happy.