One way to compare wealth is to see how many people's labor an individual could purchase. It differs across time and countries (labor is cheap in India currently for example). This book is a careful historic look by an economist - using evidence from literature, history, etc. Branko uses a variety of ways to compare individuals across history.
https://www.basicbooks.com/titles/branko-milanovic/the-haves...
At the extreme you have the "utility monster" or "freedom monster" problem. Existential Comics explores both graphically and entertainingly:
https://existentialcomics.com/comic/8
https://existentialcomics.com/comic/259
Much of Adam Smith's Wealth of Nations actually addresses the issues of inequality and the dynamic between wealth and power: "Wealth, as Mr Hobbes says, is power." That's one of the shortest and most direct sentences in a book given to long and complex writing.
The Spirit Level is a book-length exploration of the problems of inequality and highly-unequal societies.
https://en.wikipedia.org/wiki/The_Spirit_Level_(book)
https://www.worldcat.org/title/spirit-level-why-equality-is-...
Thomas Picketty's works (Capital in the Twenty-First Century and Capital and Ideology) fit into this discussion.
Oxfam have a set of suggestions as well, notably Branko Milanovic's The Haves and the Have-Nots:
https://politicsofpoverty.oxfamamerica.org/three-must-read-b...
1. Geography (which country you're born in)
2. Parents' finances
3. Your gender
These 3 explain the vast majority of the variance in income between people. Knowing these 3 facts about a person allows you to predict their income with high accuracy (small 90% confidence interval).
[1] https://www.amazon.com/Haves-Have-Nots-Idiosyncratic-History...
His "Elephant Curve" is well-know. His book "The Haves and the Have-Nots" is excellent.
I used his dataset to replicate his results to show inequality in an interactive form here:
And for a serious philosophical treatment about poverty, informed by numerous statistics comes in a long treatise World Poverty and Human Rights by philosopher Thomas W. Pogge who among other things points out the fudging of numbers by the international community. For example creating seeming amelioration of extreme world poverty by redefining malnutrition (how long it needs to persist for it to count), caloric deficit (for sedentary lifestyle rather than a hardworking farmer), redefining success (proportion of humanity rather than absolute numbers), etc. Link to relevant time in a talk by Dr. Pogge about this subject: https://youtu.be/Dsl9cyaIn-g?t=382
