I'm fascinated by interesting memos written for an internal audience - a company, a campaign or even for the President. Raw, not smoothened over for PR departments, they help shed light on how people really think inside institutions.
These are challenging to find. They typically seem to come into the public domain in one of three ways: through being really old, being part of some lawsuit/legal process or, sadly, being part of a hack.

Each of the below are 'interesting' which I define broadly as either the impact they had, the quality of their writing or just helping shed light on how communication works in a different domain. If you know of an interesting memo in the public domain (or you have permission to share), drop me a note at sriram@sriramk.com.