Although they are no longer considered official sources of Corps guidance, Policy and Regulatory Letters have value as historic references.
This page: http://www.usace.army.mil/inet/functions/cw/cecwo/reg/ contains guidance specifically for the Regulatory Program of the Corps of Engineers. The Regulatory Guidance Letters (RGLs) can be found under Statutory, Administrative and Judicial Material. To find the RGLs, click on the word "Administrative," then "Corps of Engineers Administrative Materials," and finally "Current Regulatory Guidance Letters."
The Planning and Policy page provides access to the Policy Guidance Letters (PGLs), which are found directly at http://www.usace.army.mil/inet/functions/cw/cecwa/branches/guidance/pgls/pglindex.htm.