I would like to learn something more about PostgreSQL and how it works inside. I know basics like SQL queries etc., but I'd like to know more about algorithms and functionality inside PostgreSQL so I understand this ORDBMS. For example I'd like to fully understand these slides: http://thebuild.com/presentations/uber-perconalive-2017.pdf

I would like to have a book (not a reference guide), possibly with examples and easy explanation for beginners. I can see a lot of books at https://www.postgresql.org/docs/books/ or in Google search, but I can't tell which is the one I want. Any suggestions?