Google just added a new feature to Google Book Search that lets you build your personal book library. Just search for books you own, and click the "Add to my library" link -- or, you can use the "import books" tool by typing in a list of ISBN numbers to save them all at once.
Once you have built your library you can do two things. First, you can share your libraries private URL or it's RSS feed with anyone. For example, if you want to look at my sample library, you can view it with this link or subscribe to it with this one.
In addition to this new sharing capability, you can even search your own library. For people with many books they often use for reference, this would be the perfect tool to find a specific page in the appropriate book when looking for information. It almost reminds me of Google Rooms for some reason, doesn't it?