You can learn a lot about people by finding out what types of books they like. In the past, that meant grilling them on what they had read, what they own, etc. Maybe they remember every book they have ever seen, but most likely, unless you come up with a title and/or author, they'll have no clue if you spout off about a book they love.
Enter Shelfari, a quasi social networking site that does some of the hard work for you. The co-founders, Josh Hug and Kevin Beukelman, explained at the 2007 NYC Book Expo that having a virtual bookshelf allows a person to show off your own library to anyone who wants to see it. The best part, I think, is that you don't have to give people tours of your living quarters to do it.
You can add books by searching on the title, author, subject or ISBN. You can tag books, create a top 10 list and even put your self on a blog or other social networking sites (I've included my shelf here). The interface is pretty intuitive, although I have run into a few glitches here and there.
Shelfari is a fairly new site, and with new sites, there are always new things being added. The nice thing about adding books to your shelf is that you can instantly see what other users are saying about those books. Maybe a friend mentioned that you should read a certain novel, but your friend couldn't give too much more information about it. True, you can do a web search for the particular book, but how will you remember to come back to it, when you're actually ready to read?
There are some things on the site that could be improved. From what I can tell, you really don't have the ability to change which four friends appear on your home page. A search probably shouldn't pull up every record at once, but maybe group them by 10 or 25 at a time. Overall, though, there's not a whole lot I would change to the functionality, at least, at this stage.
The only reason I mention this is a quasi social networking site is because the main objective isn't to collect friends. Your goal is to collect books. The more books you have, the more friends you'll have, right?