The Bibliothèque nationale de France is more than a reading room for scholars. Its François-Mitterrand site on the Seine is a vast modern library complex with exhibitions, public spaces, and one of the great manuscript collections of Europe.
You can visit for the architecture and temporary shows even if you are not doing research. The scale is part of the appeal: four book-shaped towers around a sunken garden, with the river and the newer eastern side of Paris just outside.