The Library of Congress has nearly 150 million items in its collection, including at least 21 million books, 5 million maps, 12.5 million photos and 100,000 posters. The largest library in the world, it pioneers both preservation of the oldest artifacts and digitization of the most recent--so that all of it remains available to future generations.
BoingBoing has a most excellent photo-essay on the Library's preservation efforts. There are also a couple of very interesting videos. Worth a look.