Books are just like pictures. They are lovely to have, wonderful to peruse, and a pain to organise. And it's immensely helpful to organise them - having keywords and tags to search pictures, and having keywords and notes to check whether a book was helpful or not. Plus a bibliography programme can help with citations - and save a major pain when writing.
There are several programmes on the market, the two best-known of which are probably
Zotero (a free plug-in for Firefox) and
Endnote (a commercial stand-alone programme).
Just recently, I have re-assessed Zotero as a possible alternative to my usual bibliography programme (which is Endnote). There are pros and cons for each of these two programmes, but
one of the best comparisons I have found yet is this one from profhacker. So, even though my version of Endnote is quite a few years old now already - from how everything in Endnote is integrated in my workflow, I'll stay with it.
If you do not have a bibliography programme yet or are not totally content with yours, though, you might want to check out Zotero. After all, it's free - so it can't hurt to try.