There is no Ink in Inkling
Inkling is an iPad textbook app through which textbooks books can be purchased, read, and annotated. It has a pleasant user interface, and (as of this writing) a small collection of what look like high school or intro college level textbooks on a range of topics. This content seems to have been either developed, or heavily adapted, for the iPad app. This makes for a smooth reading experience, loosely anchored on the book metaphor. In addition to reading per se, the app offers some standard navigation and annotation features, but these are works in progress.