calibre - Ebook management
It's a great idea to be able to convert e-books from one format to another. However, some of the key conversions (like epub to mobi) that users would frequently want are very poor and produce a product which looks substantially different from the original and is much harder to use. You have to learn which conversions work and don't work, and adjust what you do occasionally. For example, if you want to convert an epub book to use on Kindle, you would probably want it in mobi format but you should instead convert it to PDF, because that works well and the mobi is very poor.