SRE Books by Google:

https://landing.google.com/sre/books/

Real-World SRE by Nat Welch: https://www.packtpub.com/eu/web-development/real-world-sre

SRE: Measuring and Managing Reliability on Coursera:

https://www.coursera.org/learn/site-reliability-engineering-slos

Implementing SLOs by Alex Hidalgo:

https://learning.oreilly.com/library/view/implementing-service-level/9781492076803/

Seeking SRE by David Blank-Edelmann:

https://learning.oreilly.com/library/view/seeking-sre/9781491978856/