Kant's Prolegomena - Section 3 - The Possibility of Metaphysics and the Activities of Pure Reason 

a good spin off based on the synthetic apriori statement "humans act" is Ludwig Von Mises book "Human Action", and the short explanation on the Kantian methodology of praxeology(the study of human action) in Hans Hoppe's "Economic Science And The Austrian Method"

as a counterbalance to the political philosophy by the person above me, I'd recommend reading Alfred Sohn-Rethel and his concept of real abstraction, primarily in his work Intellectual and Manual Labour: A Critique of Epistemology. You're going to need prior knowledge on Marxian philosophy however, so for that I'd recommend perhaps Raymond Geuss or Robert Paul Wolff's lecture series. Of course, being a materialist Sohn-Rethel critiques a lot of these concepts constructively (as one can see from the title itself..) and its historical manifestations

Indeed

Reason is a third faculty of the mind, distinct from intuition and the understanding. Whereas the Understanding is part of the constitution of experience that subsumes intuitions under concepts, Reason pursues the unification of experience into a coherent whole. It's what's responsible for us not only being able to develop concepts, but to refine those concepts into theories and adopt the best available theories. It's also what's responsible for transcendental illusion. It does, indeed, seem to "pop up out of nowhere," but the source for this faculty (as was the case for the others) is that it appears to be a necessary condition for us to have experiences in the way we evidently do have experiences.