If you have yet to breathe the crisp, chilly, unforgivably Nordic air of Kantian philosophy, this would be the place to begin; it will prepare you for the lung-searing experience of the Critique of Practical Reason, although not for the subzero temperatures of the Critique of Pure Reason. To mix my silly metaphors even further, think of the Critique of Pure Reason as Mt. Everest; it's the most difficult and rewarding of Kant's works, at least, in my opinion. But, you have to work up to it. Okay, let's just shoot the metaphors in the head, so to speak.
Here we have Kant's preliminary ideas that would eventually lead up to the Critique of Practical Reason. Here he first puts forth the Categorical Imperative, which seeks to give a reasonable and coherent standard for ethics. According to Kant, there are three sorts of philosophy: Logic deals with the purely subjective a priori workings of the mind, while Ethics and Physics deal with both the subjective and the empirical worlds, the physical and mental realms together.
This brings Kant to what we can call the Big Problem. The Big Problem beguiles philosophers from Descartes until today, and is often called "the problem of free will". Essentially, the problem is that everything in the physical world acts in ways that are determined by the laws of nature, except us. We're part of the physical world, and yet, we believe that we have free will, and actually, if we don't have free will, all standards of ethics are meaningless. The problem, in Kant's words is that, "the freedom attributed to the will seems incompatible with the necessity of nature." The Big Problem has become even more difficult the more we understand neural physiology, because the explanation of what should constitute free will becomes even more mechanistic and in line with natural laws. We still can't quantify free will.
For Christians, this isn't a problem- free will is the imperative of the non-physical soul. For Descartes, there seemed to be a problem mostly in how mind and body work together, but he assumed there was a soul. Actually, Kant does too, but he wants to give a reasonable standard of ethics that doesn't have to explain itself by reference to God. Also, I believe that he wants to explain why Christian ethics are completely rational to the atheists in the audience.
Does he succeed? Well, according to Arthur Schopenhauer, "Aw, hell no!"* Kant switches the terms around and gives us another unverifiable explanation for ethics. The Categorical Imperative is: "I ought never to act except in such a way that my maxims could become the basis for universal law." Or, in other words, I should do unto others as I would want people to universally do unto each other. Schopenhauer's response was actually something more like "Oh? And why exactly ought I do this?"
I'm not sure that Kant ever solves the Big Problem either, although he suggests Aesthetics as the realm in which body and mind interact in the larger Critique, which I think is an interesting answer to the Cartesian problem. As one of the atheists in the audience, I don't actually think he solved the ethical problem either, although I do think he has an interesting way of justifying ethical behavior. (It should be obvious why I got so annoyed with that tool who was talking about 'Kantian nihilism'.)
As for the Big Problem (which I've probably not summed up very well), could the solution be to extend randomness to natural law? Physics during Kant's era tended to see the universe as completely deterministic- basically a big clock. Isn't that a mistake too?
*Not an exact quote from Schopenhauer.