The very interesting blog "Curious Expeditions" has a must-see post containing a large number of images of the most beautiful libraries from around the world. Here is the Strahov Theological Hall, in Prague:
Here are more images of that amazing room, from a Flickr page.
One library I would add to the Curious Expeditions compendium is the library of the Brooklyn Historical Society (completed 1881, George B. Post, architect), one of the most beautiful rooms in New York:
One more American library, this one included at Curious Expeditions:
That's the Peabody Library (Edmund G. Lind, architect, 1878) in Baltimore. It's one of the most special places in this country. Who out there has ever designed a library? Let us post pictures of it!