The Friends of the Ann Arbor District Library now have their own website - faadl.org - and info on book shop hours are available on that site. Here is the info from that site:
Bookshop Schedule: The shop will be open Saturdays 10am-4pm and Sundays 1pm-4pm. The shop opens the weekend of September 15-16, 2007 and closes after the Summer Sale, June 21-22, 2008. Preview Night (for members and volunteers only) is scheduled annually in September before the shop's fall opening.
There will be some version of a bag sale at the Summer Sale, to clear out old inventory and make ready for new.
Thanks to Tim Grimes from the AADL and Sally Allen from the FAADL for this info.