How does one manage a growing library of ebooks? By using Calibre, a free and open source e-book library management application. It has a cornucopia of features divided into the following main categories:
Syncing to e-book reader devices
Downloading news from the web and converting it into e-book form
Comprehensive e-book viewer
Content server for online access to your book collection
There are versions for Windows, Mac, Linux and Portable (to carry on your USB memory stick)
Watch the grand tour demo at this link http://calibre-ebook.com/demo
In addition to keeping track of, and organizing your ebooks, Calibre is a superb tool for converting books from one file type to another. If you've found a book you want to read, but it's a .prc file (which the Kindle reader can read), but you want to read it on an iPad, Calibre will convert it to an epub file, which the iBooks app reads easily.
If you've loaned your tablet to your little brother so that he can play games (or Skype with his friend in Australia), you can read your ebook on your desktop computer with Calibre.
Once you have your ebooks organized, get your paper book library organized with LibraryThing.