Federalist Papers
The 85 newspaper essays written under a pseudonym to sell New York on the Constitution, now the closest thing America has to an owner's manual for it.
The Federalist Papers were campaign literature. Between October 1787 and August 1788, as the proposed Constitution went to the states for ratification, 85 essays appeared in New York newspapers under the pen name Publius, arguing the case for it. Alexander Hamilton wrote about 51, James Madison 29, and John Jay 5, the last cut short by illness. The target audience was a few dozen delegates in a state where ratification was in genuine doubt. The essays are now quoted in Supreme Court opinions more often than any other American document except the Constitution itself.
The argument
The Anti-Federalist objection was that a republic could not work across a large territory. Every authority from antiquity onward agreed: republics had to be small and homogeneous or they would fracture into factions and collapse into tyranny. Madison's answer in Federalist No. 10 inverted the received wisdom. Faction, he argued, is not curable, because it grows from the unequal distribution of property and from human nature itself, and the cure of suppressing it is worse than the disease. But faction becomes dangerous only when one group can form a durable majority. Spread the republic across a huge territory with a wide diversity of interests, and no single faction can dominate; they will be forced to bargain. Size, the traditional weakness, becomes the safeguard.
Federalist No. 51 supplied the mechanism. Since men are not angels, the structure has to make ambition check ambition: give each branch the constitutional means and the personal motive to resist encroachment by the others, and the machine polices itself without depending on virtue. No. 78 defended an unelected judiciary with life tenure as the least dangerous branch, holding neither sword nor purse, and sketched the logic of judicial review a decade and a half before the Supreme Court claimed it.
What it was and was not
It is easy to mistake these essays for a neutral record of what the framers meant. They are not. They are advocacy, written fast and under deadline by two men who had lost arguments at the Convention and were now defending a document neither considered ideal. Hamilton wanted a far stronger executive than the Constitution created. Madison would spend the 1790s fighting Hamilton's expansive reading of the same text, which is a fair indication of how much the two authors of Publius actually agreed. The essays were also read by very few people outside New York during the ratification fight; other states had their own pamphlet wars.
New York ratified in July 1788, by thirty votes to twenty-seven, and only after enough other states had ratified to make the Constitution a fact with or without it. Hamilton's essays probably mattered less to that outcome than the news from Virginia and New Hampshire did.
Their real career began afterward. Because the Constitution is short and the Convention's own records were secret for decades, The Federalist became the standard gloss: the thing you cite when you need to know what a clause was supposed to do. Justices on both wings of the modern Supreme Court quote it against each other, which is only fitting. It was written to win an argument, and it is still being used to win arguments.