Well I guess I can stop looking now.
El Ateneo on Santa Fe Avenue in Buenos Aires is probably the most impressive book store I’ve ever seen. in the 1920’s it was a theater that could seat over 1000 people. It was later converted to a movie theater, then back to a theater and back to a movie house again and in 2000, a bookstore.
The ceiling is decorated with Romantic Italian frescos painted to celebrate the end of World War One.
The stage has now been converted to a cafe. On some afternoons jazz bands play there and their music echoes throughout the store.
The one drawback (for me anyways) of El Ateneo? All the books are in Spanish! The English-language section leaves much to be desired. None the less it would be easy to spend an afternoon just wandering around and gawking at the elegance of this historic old store.