Today I went to John K. King Used and Rare Books. It's a four story former glove factory in Detroit loaded wall to wall with books, books, books. It's the biggest used bookstore I've ever seen. Check out the website: https://www.rarebooklink.com/
Also, here's a video on Youtube of the store: https://youtu.be/m_QJbPQaAXA
It looks like quite a place. I'll probably never go there myself, since I don't go to Detroit and, checking the website, it seems to be more oriented to dedicated collectors than casual browsers. Still, it quite an accomplishment.
I'm sure their rare books room is a collector's place, but the 4 story main bookstore is quite open to casual browsers, with a lot of books at around $3.50-12.50 in price, depending on the kind of book.