Books on Amazon:
Interest/equality/Principle/P. Singer: also in the pursuit of our principle that we assess interests independently of people, we must, of course, take into account what interests someone has and that may vary greatly, depending on the gifts he has. But intelligence has nothing to do with the fundamental interests of people, such as pain prevention or food procurement.
Slavery/P. Singer: Considering interests: the suffering inflicted on the slave is much stronger than the slave owner's benefit.
Therefore, an intellect-based slave society is excluded by our principle of equal weighing of interests as well as cruder forms of racism and sexism, or discrimination on the basis of disabilities.
Marginal utility/P. Singer: the more a person benefits from a thing, the less he gains by extra supply with this thing. Such a principle can play a role in the balancing of the care of differently severely injured people.
Practical Ethics (Third Edition) Cambridge 2011
The Most Good You Can Do: How Effective Altruism is Changing Ideas About Living Ethically. New Haven 2015