While I was deployed to Afghanistan I did a lot of reading. With limited technological resources that was the best method of passing the time.
My girlfriend had been wonderful enough to purchase me an Amazon Kindle prior to me leaving country. I had taken the time to download every public domain free PDF book I could find, which was quite a few. However, while the Kindle is the best e-reader I have seen, it's organization and search features are some of the worst I have seen.
To help overcome this issue, I found a small program called Kindle Collection Manager; which helps organize the books on the Kindle. It is not a fancy or many featured program like Calibre, it is designed to be a super light weight method of modifying the library file directly on the kindle itself.
While the kindle can hold close to 3000 books, its OS is incapable of running at a decent speed with so much of its memory consumed. While the Kindle may be incapable of being filled to capacity with books, this program does allow be to group about 1000 books into manageable collections. I believe that should be enough books for me to carry around at any one time. The rest of my digital library can remain organized on my computer and I can swap out collections as desired.