I am impressed by Kant's moral or nominative laws- hypothetical imperative and categorical imperative as well as the three versions of the categorical imperative.
I want to grasp whatever Kant wrote. Could someone please refer some illustrative books which explain Kant's work in an easy to understand manner?
You're looking for a book that explains Kant entirely? Then you should read Kant's work itself. But then it won't be easy to understand. If you want to have something easy to understand, you'll have to accept it won't be complete. You have to make a compromise somewhere.
Anyway...
You may find this video and the others in the same series interesting: http://youtu.be/kLwiBg4sCaE - of course they oversimplify things.
For more, Roger Scruton wrote a 135-pages introduction to Kant. Also this is not enough to explain everything, but what Scruton does very neatly, besides giving a basic introduction, is showing the modern relevance of Kant's work.
If you're going to read a primary text, I would advise to read an opponent alongside. It always helps to get a better idea of the context and the issues they're dealing with. When I read Kant's Critique of Practical Reason, I read Nietzsche's Also Sprach Zarathustra alongside.
I will recommend a reading plan of the primary texts, so as to give an alternative to the answer and comments already here that suggest a more guided approach via secondary resources.
The reading list is in order: To understand where the philosophical discourse is at today, you must understand Kant. To understand Kant, I believe it is necessary to familiarize yourself with these 10 pre Kantian works of modern philosophy. After you have read these, and hopefully analyzed them with friends or through the help of very useful YouTube channels who have provided extensive and accessible commentary and analysis on these works, you can begin to read, understand, and enjoy the significance of Kant. I will end my list with Kant’s first critique because once you reach this point, you are more than ready to go on venturing to whatever paths in philosophy you want by yourself.
It depends on whether you're interested only in Kant's practical philosophy or in his theoretical philosophy aswell. The former depends on the latter for justification, but deontology as such, or Kantian deontology in particular, can be formulated without appealing to Kant's theoretical apparatus (although it is, of course, used in Kant's own writings).
Whatever the answer, you should definitely read the Transcendental Dialectic chapter from the Critique of Pure Reason before reading Kant's works on practical philosophy, religion and aesthetics (Critique of Judgement). The reasons are twofold:
The key work of Kant's moral philosophy is of course the Groundwork of the Metaphysics of Morals. It was the only work on practical philosophy that Kant initially intended to write. The Critique of Practical Reason was meant as a clarification of the relation between the first Critique and the Groundwork. The Metaphysics of Morals is an application of the Groundwork to concrete issues of, e.g. philosophy of right. Religion within the Bounds of Bare Reason discusses how religion is related to morality among other issues from philosophy of religion. The Critique of Judgement is explicitly concerned with bridging the gap between the 'realm of nature' (Critique of Pure Reason, Metaphysical Foundations of Natural Science) and the 'realm of freedom' (Groundwork, Critique of Practical Reason, Metaphysics of Morals) and it discusses how the ideas can be sensibly represented via symbolism. It's not an obligatory read if you're interested solely in Kant's practical philosophy, but it might be useful to get a fuller picture of it. There are, of course, also minor papers on various topics related to moral philosophy written by Kant, which you can find in various compilations.
As for Kant-scholars, you should probably read Onora O'Neil and Allen W. Wood to understand Kantian moral philosophy (which was, just like utilitarian moral philosophy, developed by multiple authors other than Kant). Both have also written on Kant's philosophy more specifically too, of course. Paul Guyer is an universally great Kant scholar, although you shouldn't rely on him if you're interested in very subtle distinctions or Kant's philosophy of natural science. But you can be sure he has written on anything Kant-related and can help clarify some misconceptions, even if he isn't sufficiently precise to demystify all issues. There are also more specific issues related to Kant's treatment of normativity and teleology, but I don't know of any one author which has written extensively on these problems. Good luck!
