Dawn Treader always has a fantastic assortment of used books in good shape. Their prices on used books are actually low, unlike a lot of used bookstores that sell used books at like new prices!
Their selection of Michigan related books is great (the state of, not just university of), so out-of-town relatives or friends might enjoy stopping in to look at books about the Great Lakes, or the Auto Industry.
I've found more than a few real gems here, like old science textbooks, an early 1900s guide to be being a good wife, to good old American propaganda from the McCarthy era.
The selection of more recent books is always quite good - it's right by Border's so stop in here before you stop in Border's to see if you can find what you're looking for at a much lower price.