App Annie is now tracking e-book sales for authors, publishers and other interested parties. The company says it is expanding its mobile applications analytics platform into other specialty areas with the first to be e-books. Movie and other digital content analytics might be coming soon, John Koetsier writes. App Annie's Marcos Sanchez says e-book tracking is just a "first step to expanding our footprint," adding, "It's all about digital content. We simply want to provide great accurate data."