I want to keep the best primary sources I've found for information about U.S. electronic surveillance, along with some valuable secondary sources, in one place for future reference; so here they are:
Electronic Surveillance: From the Cold War to Al-Qaeda (2006), from the National Security Archive at George Washington University; a good retrospective with many links to primary source documents.
The National Security Agency Declassified: History, Organization and Operations (2000), again from the National Security Archive, and also with many historical documents.
National Security Act of 1947 (full text, as amended); 50 U.S.C. Ch. 15 (with internal links that work).
Katz v. United States (1967), covered in my post "You Be the Judge" below.
Foreign Intelligence Surveillance Act (1978); 50 U.S.C. Ch. 36; Wikipedia article with references to codification and related cases; and the Federation of American Scientists FISA resource page.
Executive Order 12333--United States intelligence activities (1981), as amended.
DoD 5240.1-R, Procedures Governing the Activities of DoD Intelligence Components That Affect United States Persons (1982).
Attorney General Guidelines for FBI Foreign Intelligence Collection and Foreign Counterintelligence Investigations (1983).
United States Signals Intelligence Directive 18 (1993).
NSA report to Congress on Legal Standards for the Intelligence Community in Conducting Electronic Surveillance (2000).
NSA Transition 2001 report (December 2000).
ECHELON and the Legal Restraints on Signals Intelligence: A Need for Revaluation, by Lawrence D. Sloan, 50 Duke L. J. 1467 (2001).
The Patriot Act (2001); Wikipedia article, with many additional sources (frequently updated).
The Tools of Counterterrorism, PBS Frontline (2003), discussing FISA, the Patriot Act, Attorney General guidelines, the FISA Court and its opinions, and more.
ECHELON, Federation of American Scientists (last updated December 2005); Wikipedia article, with many additional sources (frequently updated).
National Intelligence Strategy (2006), General Michael V. Hayden's defense before the National Press Club of NSA's intelligence collection procedures since 9/11.
"Surveillance Net Yields Few Suspects," a Washington Post article in which a team of reporters glean more details of how the system is operating today.
"Secret Court's Judges Were Warned About NSA Spy Data," another Washington Post article focusing on the FISA court's reaction to the warrantless program.
Let me know in the comments if you come across any more good sources.