Originally Posted by
earth5
Calibre is a software program that can act as a digital library. You don't get books for free, but you can convert them to different formats. (Well, you can get them for free if you consider working around some DRMs as free). With Calibre, you can tag your books/pdfs, sort by title, author, genre, rating, etc. You can give put your books into series (if you're like me and can't remember which book in the series needs to be read next) and can add books covers to the files if they're missing. Just take the file, and drag/drop it into Calibre (there's also a wizard if you're more comfortable with that). By default, the file will be renamed and moved to the /calibre library folder, so everything's in one place. The other wonderful thing Calibre does is convert files to other formats (as mentioned above).
The formats available are: epub, mobi, azw3, fb2, htmlz, lit, lrf, pdb, pdf, and pmlz. You can convert individual files or batches of them, keeping or deleting the original file as you see fit. This means you can take a file and read it on your Nook, Kindle, and/or phone by converting it to the right file type. For instance, I convert everything to PDF and just use the PDF reader on my phone. But, books for my daughter I convert so that she can read them through the Nook player, keeping her library separate from mine. You can also sync books to your device when it's plugged in. And you can share those files without having to "loan" e-books (stupid DRMs)
Calibre is free, so there's really no reason not to try it (calibre-ebook[dot]com). There's also plugins so you can find duplicates, link to Goodreads, etc. Jeez, I sound like an ad. I don't mean to. I really like being able to convert books to PDF and get really annoyed with Amazon's proprietary-ness. Plus list formats work better for me than visual bookshelves. In short, I really like Calibre (and it's free!)
Hope that helps answer the Calibre question =)