Ethics: A Very Short Introduction by Simon Blackburn could serve as an owner's manual for your ethical decision making process. There are twenty sections, including Relativism, Birth, Freedom and paternalism, Contracts and discourse, and Rights and natural rights. One of the benefits of the book is that you can draw some guidelines from it. Some that I drew were:
1) make your own ethical decisions, but at least find out what good thinkers have said about the issues
2) look for a "course between the soggy sands of relativism and the cold rocks of dogmatism" (quote from the book)
3) make sure you're not acting purely out of self-interest
4) even though your free will isn't limitless, there are ways in which you can make decisions of your own
5) watch out for things that you really want - make sure that it's really a good thing to have them
6) know about the theories: greatest good for greatest number, the good life and person, natural rights, what if everyone did that?, social contracts
My dad, born in 1897, might have said that "do the right thing" was a complete ethical treatise. It's not that concise, but this book cuts through some of the thicket.