ZOTERO - http://www.zotero.org/

In short, Zotero is a free Firefox web browser extension that aids you in collecting, managing and citing your research sources. All happens in your web browser itself. There are tones of features for this extension but a few of them which deserve some attention are (as you may have guessed) ebook related. Some of the features that you might find interesting are:-

1. Create a mini library on your computer;
2. Store PDFs, images and web pages;
3. Take rich-text notes in any language;
4. Access your library from anywhere, even when you’re offline;
5. Automatically grab metadata for PDFs;
6. Instantly search your PDFs and notes;
7. Advanced search and data mining tools;
8. And many more …

The basic idea behind this extension is to use Zotero as your “ebook download manager” instead of your default browser download manager. It saves any ebooks to your computer in your Firefox Profile directory for offline viewing. Create a brief description and enter tags for your downloaded ebooks. Being a ebook manager, I believe this extension can be used to download ebooks in other formats as well. There is a video at the homepage showing you how Zotero works and 2 more videos once you have it installed, so you have ample instructions and idea on how to start working on it. There might be a lot of ways but from our end, you can access Zotero from your Firefox by clicking on the icon at your bottom right, from the menu (Tools - Zotero) or simply by right clicking an ebook download link and select “Save Link as Zotero Snapshot”.

Check out this extension by clicking on the link below:-
http://www.zotero.org/