All of Kant's works are difficult reading. On the other hand, because he was so influential, anyone who has done any general reading on modern western philosophy has encountered some of his ideas already--and, as a tutor and professor for more than fifty years, he was certainly adept at leading his audience through a train of thought. He may not dumb anything down, but if you stay the course and keep reading, it does all make sense eventually.
The core of his work are his three critiques: the Critique of Pure Reason is the single most influential work of western philosophy, and has the size and depth to back it up. It's helpful to read a primer on David Hume, or an edition with an ample historical introduction, but most of the book actually reads just fine on its own, which is one reason it's so long. Not as commonly found is the Prolegomena [which just means "prologues"] to Any Future Metaphysics, a restatement of Pure Reason's main points and a response to some of the criticism against it. The second critique, The Critique of Practical Reason, deals heavily with moral philosophy and argues that a proof of God's existence is impossible--not because God does not exist but because he is by his nature unknowable and beyond our ability to prove. The third critique, the Critique of Judgment, may seem almost frivolous compared to the first two, but it laid the groundwork for the philosophical pursuit of Aesthetics. By discussing the human ability to judge--for instance, to determine that something is beautiful or good--Kant discusses the nature of the human mind and the faculty of genius.
