Epub.js
Enhanced eBooks in the browser
...Just include a single javascript file, and pointing it at a hosted epub. Epub.js provides an interface for common ebook functions (such as rendering, persistence and pagination) without the need to develop a dedicated application or plugin. Importantly, it has an incredibly permissive Free BSD license. The EPUB standard is a widely used and easily convertible format. Many books are currently in this format, and it is convertible to many other formats (such as PDF, Mobi and iBooks).