The Jefferson Building of the Library of Congress is probably one of the most impressive buildings in Washington DC. Because of the scale of the architecture inside, it is hard to photograph without a wide-angle lens (which I didn't have at that time), so you probably won't get a feel of what it is like to visit this building in person.
I wanted to do a little genealogy research, so we went and got reader's cards so I would be able to view materials. The whole process is not very time-consuming and you leave with your card in about half an hour (my picture was horrible, which is why it isn't here!!!). I did find the books I was looking for, but unfortunately they didn't have the information I was hoping for...
We also took the tour, which I would recommend. The knowledgeable staff will give you a detailed look at the Library and tell you some of its history.
Some of the ornate decorations of the walls and ceilings
(Yes, that's real gold)
The main reading room
(This picture doesn't do it justice!)
A mosaic of "Astronomy" from the Sciences section