The reference section in Mullen Library is a great place to start your research. These titles will give you a great introduction to many topics related to Washington, DC. Be sure to check out each source's bibliographies to add depth to your sources!
If you are interested in Presidential papers, be sure to check out the J80s range in the general reference section in Mullen Library. There you will find the public papers of many of the Presidents.
[image from flickr user Scott Ableman]