For $1,999, Apple will now let the stranger across from you on the train think you're reading The Great Gatsby. Hold the new foldable iPhone like a book and that's what happens. It also undoes the thing that turned Fifty Shades of Grey into a monster hit 14 years ago. Here's how it works. You fold the iPhone Duo partway and it sits in your hand like a paperback. Inside, Apple Books puts two pages side by side on the 7.6-inch screen. On the outside, on the 5.4-inch screen facing the rest of…