I remember that there was someone who wrote down the stuff the founding fathers talked about behind closed doors. Can't remember his name. Who was he again?
Assuming you are talking about the Constitutional Convention...
It wasn't a "secret", but I suppose it is conveniently ignored by a lot of people that there were extensive records kept of the deliberations. They were delivered by the convention secretary to the convention "president" (George Washington, of course). Congress ordered them all printed in 1819.
Max Farrand in 1911 compiled them all and reprinted them together (in multiple volumes) as The Records of the Federal Convention of 1787.
I haven't read them myself directly, but they were from multiple sources, not just official proceedings notes. According to the link above, aside from the official notes the largest amount of material in there was written by James Madison. From the secondary sources I've read, it seems like rather a lot of the material in there was originally written by Alexander Hamilton as well.
Charles Thomson was the secretary of the Continental Congress from 1774 to 1789. In addition to keeping the official journals of the Congress, Thomson kept a number of private notes, which in later years he attempted to compile into a manuscript. This work, tentatively titled "Notes of the Intrigues and Severe Altercations or Quarrels in the Congress", reportedly reached over a thousand pages in length. Unfortunately for historians, Thomson eventually decided against publishing the manuscript, and instead destroyed it.
Nathaniel Philbrick wrote about Thomson and his manuscript in the preface to his book Valiant Ambition, which is how I learned about it.
In 2011, the New York Historical Society acquired the Constitutional Convention notebooks of John Lansing, Jr., a New York delegate to the 1787 Philadelphia Convention. According to this announcement:
The delegates’ vow of secrecy, which banned the taking of notes for publication, limited the amount of material created documenting the Convention proceedings. Although notes by a number of other delegates, including James Madison, survive, Lansing’s are among the purest and most detailed, providing a unique and unedited first-hand account of the period of Lansing’s attendance at the Convention.
The society also holds the notes of Massachusetts delegate Rufus King, and has on deposit those of South Carolinian Pierce Butler.
Beginning on October 27, 1787 the Federalist Papers were first published in the New York press under the signature of "Publius". These papers are generally considered to be one of the most important contributions to political thought made in America.
The papers were actually written by James Madison, Alexander Hamilton and John Jay, all under the single pseudonym Publius as noted above. They were never secret, and were published as they were written as part of the campaign to achieve ratification of the Constitution.