Whether you are someone who is "thinking of writing a book someday" or a published author, you need to be concerned about having a platform (great book by Michael Hyatt) and community ready to support your book.
Community Building Ideas for Authors
Start an author blog - share your thoughts and tidbits from the book (Hoop.la would allow you to share blogging responsibilities with others as well.)
Share your book tour dates in the calendar - Hoop.la's calendar is compatible with iCal, and includes maps and RSVP capabilities for events like book signings or appearances.
Integrate your social feeds - if you're already connecting with your audience on Facebook, Twitter, or other social networks, you could have widgets that pull in your social feeds to the Hoop.la home base.
Include ads or promotions - Hoop.la would allow you to easily place graphic ads or promos for the book release within the community, linkable to your Amazon page and elsewhere.
Forums would expand the conversation - once you have a nice amount of traffic on your site, you could open forums and allow your readers to discuss the larger issues surrounding your subject area. This would extend your reach beyond simply the book release, and perhaps lead to your next adventure, whatever that might be!
That's just the tip of the iceberg--Hoop.la gives your fans and readers a centralized place to go and support each other, and help support your book as well.
Now go start writing!
Photo credit: Via Flickr Creative Commons, miamism