Deduction of the pure concepts of the understanding (i.e. the categories). HERE. Wolff mentioned some recommended reading, for Kant I'd add Stephan Korner's short but masterful Kant (Korner was from Yale and incidentally taught Robert Hahn who at SIUC led the Kant seminars I went through).
I would add H.J. Paton's Kant's Metaphysics of Experience Vols. I and II.
Throughout my graduate education I took several Kant seminars (essentially working through all of the Critiques), but when it came to the first Critique Paton's book(s) were invaluable. Just as anyone studying the Phenomenology of Spirit requires Harris' two volume Hegel's Ladder, anyone studying the first Critique requires Paton's Kant's Metaphysics of Experience. Nowwhile the latter is in paperback with a decent price it is justifiable to argue that even in hardback in excess of $100 both of these texts are worth it considering they are the "gold standard" for learning one's Kant and Hegel.
(For more on required Hegel reading see HERE and HERE.)
