C-SPAN have recently broadcast a programme about the Library of Congress. The documentary is available here. It tells the story of the institution which began as a reference library for Congress, but has grown into one of the most important centres of information and culture in the world. It is the largest library worldwide in terms of shelf space and the number of books. I strongly recommend watching this fascinating documentary and visiting the Library's website here. In addition to the books there are collections of recordings, maps, newspapers, and manuscripts.
The Library also contains a research service, the Congressional Research Service. I've been impressed by both the quality and quantity of material produced by CRS. Their reports are prepared for Members of Congress - but thanks to OpenCRS, documents already in the public domain can be accessed at the OpenCRS website. Whether you are looking for something on practice and procedure in Congress - or a report on a specific policy subject - CRS in one of the best places for an impartial, well researched - and well written report. These can be short articles or long, in-depth pieces.
The Library is spread over a number of buildings. CRS are housed in the Madison Building, while the most famous of the sites is the Jefferson Building - worth a visit just for the superb art it contains. (It also has an excellent bookshop). The main reading room is a superb place to research - not only because of the wealth of material available in the Library - but for the beauty of the room itself.