Key features include; supports a wide variety of eBook formats such as EPUB, PDF, MOBI, CBR, CBZ, among others, allows you to add comments or tags to books for better searching, ability to download RSS feeds or news from sites and then convert them into eBooks. The application is relatively lightweight and easy to operate. Plus, it offers a highly intuitive user interface.
Calibre is completely free to download and use. Additionally, it is compatible with Windows, Mac, and Linux operating systems.