Sigil is a free, open source, multi-platform ebook editor. It is designed to edit books in ePub format.

With Calibre, you can transfer books between your computer and e-reader, and vice versa, converting their format for whichever device you choose, and removing the DRM so that they can be shared across all of your devices instead of just one. Additionally, the app can store your library online so that it can be accessed by you from anywhere in the world, making Calibre probably the most powerful e-book management software available right now.