I love the old book stores in Paris such as Shakespeare & Company but one that is on the top 5 list is Taschen. The company is based in Cologne, Germany and is known for its art books. They have stores in some large European cities as well as some in the USA, such as NYC, LA, Miami and Dallas.
I love the store because the books are always amazing, very colorful and for the quality of them, which is excellent, they are very inexpensive. Also, they do a great high quality store catalog which I pick up and seem to never throw away.
In Paris they whell out a couple tables so that you can browse the books in the sunshine.