If you are anything like me, you have a ton of various eBooks lying around on your hard drive. For a couple of years, I have been using Calibre to keep mine organised.
Calibre is a piece of Freeware, although they do ask for donations. It is by far the best solution I have found of keeping track of your Ebooks. It also allows conversion between different formats, which works pretty well 99% of the time.
The application is very actively developed, and has recently come out of Beta, although it has been stable for at least the last two years. There are regular updates, about one every couple of weeks.
So, if you are an avid reader, like yours truly, get Calibre.