The questions you are asking are usually classified as "meta-ethics".  Meta-ethics asks about things like the nature of moral propositions, the knowability of moral truths, and other various topics that are focused on the nature of moral truths and systems. 

For whatever reason it seems that most philosophers who ask questions like this get mired in questions about ethics without ever arriving at ethics, if that makes sense.  Since we all have a sort of morality and all live in a world steeped in moral language and judgments, I would recommend starting your moral inquiry from that point of view, as opposed to something like metaphysics or epistemology where it seems easier to start from a purely abstract and theoretical point of view.  This was Socrates' method.  For example it might be easier/more useful to start by reading a text on virtue ethics or utilitarianism than by looking for a systematic text on abstract questions about the foundations of norality.

As far as a text dealing with the questions you are asking about, which are indeed good questions, I will let others offer recommendations.  None of that is to discourage you from asking about meta-ethics.  Just offering my teo cents.