From ArborWiki

Aunt Agatha is a bookstore located on Fourth beween Liberty and Washington. It specialies in mystery, whodunit, and dective novels. In-store book signings are not uncommon.

213 S 4th Ave, Ann Arbor, 48104 - (734) 769-1114

Aunt Agatha's website: http://www.abebooks.com/home/AUNTAGATHAS/