Calibre is a very comprehensive e-book management tool. It has three main functions: management of your e-book library; conversion filters between different formats, and e-book reading capabilities.
The management function allows you to store your books in one place so you don't have to search them each time you want to read. Also, this feature lets you edit the metadata of the book so the information about your e-book is complete and accurate.
The conversion feature is perhaps the most characteristic of this program. As its name implies, it lets you convert an e-book from one format into another so you can share them between readers. For example, if you have an e-book in the Kindle format but you want to read ir in an epub reader, you can convert it into that format using Calibre's feature.
Finally, the e-book reader lets you open and read e-books regardless their format. It also lets you change the size and type of font, read your book in full screen, add bookmarks, print the document (when allowed), and so on.
The program also includes an e-book search engine that connects with the main e-book providers such as Amazon's Kindle Store, Barnes&Noble, and more to search for a particular book, theme or author. Then, the program redirects you to the provider's website so you can purchase the e-book.
In general, this is one of the most comprehensive e-book management programs, and also one of the most popular. If you enjoy reading e-books, then you may find it useful.