A favorite of mine. In a nutshell, Paco Underhill and his researchers spent 20 years in stores observing interaction between shoppers and store. He identifies the "butt-brush factor" (bumped from behind, shoppers become irritated and move on). They've quantified the importance of shopping baskets, "circulation patterns" (how shoppers move throughout a store) and much more.

This is not a new book, but is a wealth of knowledge I think is applicable to all kinds of businesses—not just retail.