I had to travel back to Michigan a few months ago for a short weekend and was lucky enough to take a few hours and stop by (spend 4 hours in) one of my favorite bookstores in existence, John K. King Used & Rare Books, in Detroit, Michigan. The bookstore resides in an abandoned glove factory and is said to have about 1 million books in stock.
If you ever find yourself near Detroit, this place is top of my list for checking out. It's been a tradition in my family to go to this bookstore and the minute I walked in, the small of old books transported me back in time.
My father, who once worked for Campbell Ewald (now Lowe Campbell Ewald), heard that because they recently moved buildings, they had to also downsize their library. King's had purchased the library and was slowly selling their way through it. Obviously, we made a b-line for the graphic arts section. We spent about 4 hours there. A whole day wouldn't be enough for me to get through the whole place. I had barely touched the astronomy section after completing the graphic arts section when we had to leave.