One of the best ways to learn a new language is by reading books in it. But this can get very tedious, as you frequently need to switch between the dictionary and the book you're reading. Duofolio's purpose is to make that experience a little bit easier. It has a built-in dictionary that lets you find out the meaning and listen to the pronunciation of foreign words without breaking your concentration.
Duofolio currently supports 4 languages with more coming soon:
Search book contents
Can be used as a regular ebook reader