We did our best to highlight the intersection of security and reliability. I admit there might be more emphasis on security in some chapters, especially those that overlap with the previous two SRE books (that were exclusively about reliability). We wanted to avoid repeating ourselves.

(disclaimer: I work at Google)

This is correct. This is the third book in our SRE series. It's reliability through the lens of security.

Disclaimer: I work for Google

What is the focus of the others?

Much more general.

The first book is a solid overview of how Google does SRE and outlining each of the various concepts (error budgets, blameless culture, etc..). The second is more of a practical guide on deploying SRE into an organisation, a lessons learned type of book.

(I work for Google but not SRE, just enjoyed reading the books)

Why are 51% of your people contractors?

It is. You can just take a look at the table of contents.