My fantasy is based on the library at the Lutheran Seminary in Germantown, Philadelphia, PA, where I've gone to do genealogical research. The building is of stone, like a cathedral. There are enormous windows that go from the roof almost to the ground. The interior is essentially a cavernous space within which the floors and bookshelves are an integrated steel structure, with the floor tiles made of glass to let the light through from one floor to the next. There are also fluorescent lights, but for a multi-story library, the amount of natural light is extraordinary.
It also has an impressive, circular, stone atrium where they put choice texts and historical documents on display. Furthermore, the staff is enormously knowledgeable and eager to help.
My plans are a little simpler than that, with about four thousand volumes to house - shelves all around the room to a height of five feet or so (since I don't like being up on ladders), then another four feet of empty space above to house our growing collection of World War I posters, gallery-style.
A fireplace with a large stone hearth would showcase the pride of the collection, which was done by Joseph Pennell for a Liberty Loan campaign:
http://www.english.emory.edu/LostPoets/Liberty-note.html