Books on Amazon:
Subjectivity/ethics/P. Singer: The thesis that ethical principles were based on personal subjective attitudes is a heavier ((s) more fine-grained) form of relativism, for example, if someone says it is bad to harm animals, he expresses only his opinion, which would mean that there is no place for ethical disputes.
Subjectivity/Subjective Ethics/Subjectivism/P. Singer: subjectivity is based on the fact that it is possible to think about ethics.
Problem: it could be that we think about ethics without understanding how this can happen at all. We must not lose the basics here.
Practical Ethics (Third Edition) Cambridge 2011
The Most Good You Can Do: How Effective Altruism is Changing Ideas About Living Ethically. New Haven 2015