In order to understand Kant, first read David Hume's Essay Concerning Human Understanding. Kant's major ideas are entirely a response to this essay (we English-speakers are lucky to have this crucial piece of the Enlightenment in our native tongue). Hume argues that cause and effect are entirely empirical concepts, which has the implication that we can't actually talk about "eternal laws of nature" with any sense. Kant wrote The Critique of Pure Reason and his subsequent critiques in the trilogy to argue that the laws of nature are laws because they are the laws of our ability to experience subjectivity at all. The Critique is very dense and technically written and the English translations do little to abate this. I would recommend reading it with a companion commentary text though unfortunately that wasn't the path I'd taken so I can't pick out a specific one.