Who: Kovid Goyal
When: Right now!
I'm into ebooks, read them on a Kindle and use Calibre to manage them all.
This software does everything. It converts between ebook formats, read ebooks, edit the meta data and change cover images. You can even configure Calibre to email books to your device. This feature is particularly useful with the Amazon Kindle.
There is really nothing that this program cannot do. Head on over the the Calibre website and check out the features.