Web developers are constantly looking for a niche to fill – a focus for a site that will attract a reasonable number of users. Some social sites are general and some encourage interaction around a topic or process. I would argue that Facebook is an example of the general approach and users can shape the general tool to their interests. In contract, a social bookmarking site (e.g., Diigo) is organized around a process (personal bookmarking) that has a social element (sharing bookmarks).
The books we read can become the basis for social interaction. Face to face groups that agree to read and then discuss a given book are common. In a way, this focus and the potential for a similar interaction has found an online niche.
I have two examples of this category of services. Both allow users to identify the books they have read, are reading, or want to read. Both allow comments on these books and opportunities for interaction. I have treated these services like I have treated the social music services I use (e.g., LastFM) – spend some time with the record-keeping capabilities of the service, but I ignore the social component. I am not interested in sharing concert experiences related to the musical groups I follow (I have no such experiences) and I am also am limited in the time I can spend interacting online so I mostly post comments on the books I read to my blogs rather than the book sites. So, I am not a poster boy for these services, but perhaps you find this focused topical sharing of interest. I do read the comments of others related to some of the books I have read.
So my two suggestions if you are interesting in exploring social conversations around books are:
