Publisher's or author web sites. If a publisher doesn't have a library marketing department call the publicity department and ask for their latest catalogs which will list current and upcoming authors with planned tour cities.
Points to consider when selecting an author: audience size (a track record of attracting a large audience will give you an advantage in securing a "big name" author), meeting room size (smaller rooms are good for first novelists and mid-list authors).
Updated 02/02/2012
Acknowledgement: to the Dakota County Library (Eagan, Minnesota) for
some ideas and information used in this material.