Social Security Office of the Actuary: Proposals Addressing Trust Fund Solvency Invaluable. Has scores for proposals as recent as the 'bi-partisan' Commissions and as old as 1995. Ever wonder what the Ball Plan, or Pozen, or Ferrara, or LMS, or the Ryan Roadmap look like from the standpoint of SSA's Chief Actuary, and with plenty of numbers, this is the place to look.
Congressional Budget Office Publications: Social Security and Pensions. CBO publishes a Long Term Budget Outlook plus a separate Long Term Projection specifically for Social Security almost every year plus periodic other reports and studies. Links to these publications back to 1977 available from this link.
The SSA.gov History page has much more than narratives, it also has links to the texts of the various Acts back to the original Social Security Act of 1935 plus legislative histories. Truly a fantastic resource Social Security Administration: History
Social Security News. Stories from the front lines of the battle to make Social Security work day in and day out, by people inside and outside of the Social Security Administration. (I have a friend who recently retired as a field administrator from SSA and she swears by it. It is a little 'inside baseball' for those not directly involved in Social Security administration or appeals.)