This is a selective guide to the official government documents related to the terrorist attack on the World Trade Center in New York, NY, on September 11, 2001. There is also a more general research guide on terrorism available, titled Terrorism: Information Resources.
You may also want to consult Understanding 9/11: a Television News Archive from the Internet Archive or The September 11 Digital Archive, two collaborative efforts by libraries and archival institutions, which have compiled hundreds of images, collections, and web sites which reported on the September 11 attacks. Both archives can be searched in a number of useful ways. The Library of Congress has aggressively acquired a vast range of materials related to the attacks on September 11 in its collection, Witness and Response. The New York Times has published an extensive series, Portraits of Grief, which presents short biographical profiles of many of the victims of the September 11 attacks.
Finally, there is an extensive two-volume bibliography, Annotated Bibliography of Government Documents Related to the Threat of Terrorism & the Attacks of September 11, 2001, compiled by Kevin D. Motes, Oklahoma Department of Libraries. It is much broader in scope than this guide, and includes much general background information.
Note: This guide is not being updated. Most of the documents listed were issued 2001-2006, although there are a few titles which were published more recently.