Google SRE guides are great overall, but I caution new people to the industry to be aware that some of the advice is attuned to Google's needs - not necessarily your org's. Complexity is the enemy of scalability. Your startup's XYZ service doesn't necessarily need to be using a distributed database or front-end caching or fancy QoS right out of the gate. Ship early and often and the start of your SRE career will come easier.