i.e. The Critique of Pure Reason. (Kant wrote 3 Critiques) It's known as one of the most difficult-to-read philosophy books in history, though, so don't expect to pick it up and understand what he's saying. I did an entire university course on the book—I mostly studied philosophy at university—which was gruelling, and sometime later when a girlfriend saw the fat book on my shelf and asked what it's about, looked horrified when I couldn't tell her. Never heard anyone say he was 100% right, though. Particularly his own successors (Neo-Kantians) who, I believe, thought important parts of his system should be dropped—mainly, Kant's reality/the noumenon/thing-in-itself that we can never know or say anything about.
(The most helpful link I can think of is James Franklin's article on Stove's Gem, the "Worst Argument in the World"—Very lucid writing, and not a bad introduction to talk of things-in-themselves. https://web.maths.unsw.edu.au/~jim/worst.html )
Neo-Kantian generally refers to the application of the style and epistemology provided by transcendental idealism in treatment of other subjects, rather than as a departure from Kant.
The Thing-in-Itself was once controversial but undeservedly. It's necessary to prevent the subject knowing the object fully, so thus it is treated in the sense of Cassirer/Cohen's Infinitesimal. No one has trouble with Noumenon maybe only in the disappointment with the intractability of metaphysics to answers some of the most pressing questions.
Yes, so difficult to read that he wrote the much more accessible Prolegomena a few years later to summarize his ideas. Critique if Pure Reason is a classic, but what bootstrapped his ideas into the public academic society if the time was Prolegomena.
Prolegomena is the introduction to his Critique of Pure Reason, published after that work because he realized he needed a something more accessible to introduce his concepts. Condensed, it stands as a summary, and thus still his response to Hume. That response in fact is a critical part of the opening pages of Prolegomena. My statement was correct.
Hume would not have considered the analytical response as adequate indeed neither does Kant. Only a synthetic treatment of the subject of logic and causality itself could resolve the Scottish Skepticism--that's why these books do not have a comparable degree of profundity. It is the synthetic movement from experience to intuition and back to concept which is the great achievement. The analytical treatment provided by Prolegomena does not afford this.
It is not intended as an introduction to Critique of Pure Reason it works from the conclusions of that work and why those conclusions are necessary.
We can remember the title isn't Prolegomena to Critique of Pure Reason but instead Prolegomena to Any Future Metaphysics.
(The most helpful link I can think of is James Franklin's article on Stove's Gem, the "Worst Argument in the World"—Very lucid writing, and not a bad introduction to talk of things-in-themselves. https://web.maths.unsw.edu.au/~jim/worst.html )