BOOK: "Extreme Programming Explained: Embrace Change, 2nd Edition" (The XP Series) - Kent Beck

The definitive guide to XP is the book by the author (Kent Beck):

https://www.amazon.com/gp/product/0321278658?ie=UTF8

Extreme Programming Explained: Embrace Change, 2nd Edition (The XP Series)

2nd Edition, ISBN-13: 978-0321278654, ISBN-10: 0321278658

PS. Additional info about XP, considered the first of the agile methodologies (long before Scrum):
https://martinfowler.com/bliki/ExtremeProgramming.html

Methodologies: Test Driven Development

https://martinfowler.com/bliki/TestDrivenDevelopment.html

Quoting:
"Test-Driven Development (TDD) is a technique for building software that guides software development by writing tests. It was developed by Kent Beck in the late 1990's as part of Extreme Programming. In essence you follow three simple steps repeatedly:

Write a test for the next bit of functionality you want to add.Write the functional code until the test passes.Refactor both new and old code to make it well structured.

You continue cycling through these three steps, one test at a time, building up the functionality of the system. "

The book (by Kent Beck):
https://www.amazon.com/gp/product/0321146530?ie=UTF8