Mashable.com has a list of the 100 best authors on Twitter.
Here is an excerpt from the introduction on that page:
Twitter is a great way to connect with others who share your interests or offer information you value. There are Twitter users covering virtually every niche out there. Writers and authors are no exception. Below you’ll find a listing of more than 100 authors active on Twitter.
http://mashable.com/2009/05/08/twitter-authors/
