What book(s) cover the concept of equity?

[PokrLikeItsProse]

I'm not looking for the "best" book. Ideally, I would like several books that approach the concept at least somewhat differently so that I can come up with my own understanding of the concepts involved.

I would also be interested in books that describe equity or similar concepts in other fields besides poker. Or a suggestion of what might be a similar concept in another field so that I can search on my own.