After returning to Leiden, De la Court entered his father's profession and set up a cloth trading firm with his brother Johan.
Pieter was married again in 1661, this time to Catharina van der Voort,[2] the sister of two wealthy Amsterdam merchants and, again, a relative of Johan de Witt.
It was in this turbulent period of De la Court's life that he published almost all of his books about the political economy of Holland and the larger Dutch Republic.
In the preface to the Interest van Holland, the most renowned of these books, he explicitly ascribed this publishing frenzy to the need to distract his mind from the tragedy that had hit him.
De la Court's second wife Catharina van der Voort (1622–74) had two children, Magdalena (1662)[3] and Pieter (1664–1739), later named Pieter de la Court van der Voort, who became later known as a greenhouse gardener and the most successful exotic fruit grower of his time.
There De la Court expanded the scope of his business activities by participating in the ventures of his two brothers in law.
De la Court's publishing activity had made him a well known protagonist of the republican "party" in contemporary Dutch politics.
This group, consisting primarily of the wealthy businessmen in the cities of Holland and led by Johan de Witt, effectively ran the Dutch Republic from 1650 until 1672.
They strongly opposed the political powers and ambitions of the House of Orange-Nassau which usually held the office of Stadholder, a pseudo-royal position that somehow survived in the otherwise republican institutions of the Dutch Republic, except during this First Stadtholderless Period.
It has been established as a fact that Johan de Witt and a number of other government officials contributed to the Interest van Holland.
De la Court identified free competition and the republican form of government as the leading factors contributing to the wealth and power of his home country.
Both books contain vivid albeit rather one-sided descriptions of the damage that had been done to the country by monarchical leaders since the Middle Ages.