Do you want to improve Knowledge about SRE from creators, yes from Google for free? Yes, you can get google knowledge about SRE concepts for free from Google.
If you prefer read SRE books online then all free SRE books at Google SRE books page
But if you prefer to read offline, then you can buy them online. My request is all who wants to be SRE, the first step read above 3 Google SRE books.