Manage Your e-Book Library With CalibreCalibre is an open source e-book management system that's simple to use, and offers tons of features, both natively and through plugins from third parties. It allows as much or as little customization and manipulation of your e-book files as you'd like, and is actively supported and updated by developer Kovid Goyal. It's one of the few open source programs that I donate to regularly to support its continuing development.
Calibre is installed on your computer. Mac, Linux and Windows operating systems are supported. From there, you can add your books from various sources, and edit the details of the metadata with ease, which really helps keep things organized with very little fuss. Calibre also allows you to convert your ebooks to other file formats if they don't have digital rights management (DRM) encoded.
You cannot convert ebook files that are DRM protected. While Calibre does not natively support stripping DRM from e-book files, there are third party DRM removal plugins that are easily installed that work great. When you strip the DRM, you can then convert the e-book file format to one that's compatible with whichever reader you are using.
As a Nook owner, this allows me to purchase from any vendor I want, and convert to compatible epub files as needed. By doing the conversion in Calibre, and syncing the converted files to Aldiko on your e-reader (more on this reader app below), it's not necessary to switch between dedicated Kindle and Nook reader apps. This is a much easier way to keep track of what you've read, and what you want to read next, since everything is all in one place at one time.
Calibre also supports PDF files, which is really useful if you find you have a lot of them. It's nice to be able to read some of the larger multi-page pdfs on an e-reader screen than having to either print them out or fire up the laptop.