Seldasen presents several different viewpoints or frameworks that Heralds can use for reasoning about ethics, which for him seems to boil down to 'how to make decisions so that the world around you will end up being a nice place to live'; he says that ethics is relevant to almost everyone, whether they're running a kingdom or not, but especially important for Heralds because of the responsibility they bear - because the people of Valdemar trust them and defer and listen to them about what will make their lives and their children's lives good.
You can think about ethics as a matter of principle, which is somewhat different than a matter of law, because there are ethical principles that aren't and shouldn't be encoded in Valdemar's official law and punished by the courts. Lying is wrong. Stealing is wrong. Murder is wrong. All of these can be cashed out more concretely - it's not just 'because', the most straightforward summary is that actions are wrong in principle if you wouldn't want your neighbours or your king doing them to you. Most people do not want to be murdered, or lied to, or stolen from, and so doing these to the people around you is predictably doing them a disservice, and this is - more universal and less subjective than matters of preference that vary, like favourite foods. Seldasen thinks this is a reasonable framework for Heralds to use most of the time; it's not generally confusing, most of the time, and drawing clear bright lines makes it easier to avoid rationalizations. The difficulty comes in when different principles conflict, which will inevitably happen.
You can think about ethics as a matter of virtue; to be ethical is to be kind and generous and loyal and brave and to keep your promises, and such and such. Seldasen thinks this framework is helpful when you're trying to figure out what you should do and not just what you shouldn't do. Heralds need to do a lot more than not lie and not steal and not murder people. That being said, different virtues can also conflict, and the framework doesn't in itself give a strong sense of whether being loyal is more important than being generous.
You can think about ethics as entirely grounded in the people you're being ethical for. The ethical course of action is what saves lives, or what makes people wealthier, or what prevents crime and disease and hunger and fear. This frame is in a sense a lot simpler than either of the others; you won't run into conflicts between different principles or virtues, there is nearly always a fact of the matter about what saves lives. The difficulty is that any course of action has many effects, in the immediate and medium and very long term, many of which are hard to predict in advance, and so it's easy to, with the best of intentions, do far more harm than good.
And then he goes through so so many examples of hard choices, where it's not clear what to do and different ethical frameworks might give different answers. Seldasen adores controversial frustrating examples. He never claims that there's one true right answer, here. Sometimes it's obvious from the writing that he leans a particular way, but not always.
Murder, for example: generally wrong. Saving the life of an innocent person: generally good. But imagine yourself besieged in a city, at war, and an innocent woman - a citizen of Valdemar - and her child, ill with plague, begging to be let in. Or even climbing the walls. The city has Healers; they might well be able to save the child's life without risking the lives of others. But they might not. The woman and her child might die anyway and spread disease to the besieged soldiers. She might turn out to be a spy. There is always uncertainty. But in these conditions, it's at the very least not obviously wrong to refuse to save one person, in exchange for saving many more, and perhaps even to kill to woman in order to prevent her getting in.
There are a lot more examples. Is it ever the ethical choice to disobey a superior's orders. To conceal information you know about a crime committed. To lie to someone who trusts you. Seldasen admits readily that many of the examples are carefully chosen to be as difficult as possible, rather than because they really happened, but life is often unclear, he says, and some of the examples are real ones that occurred in his life, or to his friends. (He doesn't specify which of them.)