T. M. Scanlon wrote What We Owe to Each Other, the ethics book featured in The Good Place. Scanlon is an American philosopher best known for developing a major modern form of contractualism.
The book was published by Harvard University Press in 1998. Its central concern is how people can justify moral principles to one another, especially principles that no one could reasonably reject. That framework becomes a natural fit for a series built around questions of moral responsibility, ethical improvement, and what people owe to each other.
In the show, Chidi uses Scanlon's work while teaching Eleanor ethics. The title also appears as the name of the sixth episode of season one, and Scanlon's ideas return in later episodes, including the series finale. Eleanor's eventual engagement with the book marks how far she has progressed from avoiding moral responsibility to thinking seriously about it.
Scanlon is not the same philosopher as Kant, Kierkegaard, or Philippa Foot, although their ideas belong to the broader philosophical traditions discussed by Chidi. The exact answer is therefore the contemporary philosopher T. M. Scanlon.