As an informal cheerleader for the Library of Congress, I came up with the bad idea of making a blog about why the LoC is so great, especially for researchers. Here is the LoC's general Researcher Webpage. The access to books and journals from all over the world and from centuries gone by is phenomenal.
To get started, one needs to get a new library card, which takes about 2 minutes. Just go in the main doors of the Madison Building (the most modern of the three buildings, closest to Capitol South Metro) and turn left for the registration room. There are three buildings (from oldest to newest): Jefferson, Adams, and Madison.
Unless you want to use particular collections such as Manuscripts, Prints and Photographs, or Motion Pictures, there are are basically three locations in which researchers work on a daily basis:
1) the Main Reading Room in the Jefferson Building
2) one of the area studies rooms (African & Middle East, Asian, European, Hispanic)
3) the Science and Tech reading room in the Adams Building.
The Main Reading Room is the most impressive space, but it is seriously cold. The area studies room are smaller, warmer, and often beautiful as well, and the staff are much more accessible. The European room even has a weekly tea on Fridays at 2:30pm. The Main Reading Room is open later and on Saturdays, but you can send books from the area studies rooms to the Main Reading Room, if you want to continue working after the area studies room have closed. The main positive about the Science and Tech room is that you don't have to put away your bag in the cloak room. Most academics don't choose to work there, but I find that it is good for social sciences (the social science books are in that building) and it requires less commitment since you get to keep your bags, coats, etc.
When you order books (which can be done online once you get a new library card), you can hold those books on the 3-day reserve shelf. These books have to be re-reserved after 3 or so days. Everyone can apply for a shelf, which allows you to store books indefinitely, but I think that you have to show that you have come into the Library once a month or something.
There is a cafeteria on the 6th floor and the basement of the Madison building. There is a cafe with great staff next to the basement cafeteria. There is also a small cafeteria on the first floor of the Adams building. Pete's Diner across the street has a great pan-Asian veggie combo that changes every day.
