I have a Sony 505 eReader that I use to read a wide range of information on.  Shortly after my eReader purchase I download calibre for managing my documents.  It seems that every couple of weeks there is a new version too.  Each version does a good job of improving on the last.  The program itself is quick, it is written in python.  The interface is intuitive and has some nifty features.  My favorite is that it can grab the contents of a site and quickly convert it to a format friendly to my Sony.  Another nice feature is that it can easily convert books between formats.

I encourage you to try it out if you have a reader or lots of books.  It also has a built in viewer.  Download here.