I'm currently reading "Designing Data-Intensive Applications" by Martin Kleppman and it has an interesting chapter on database storage engines, including Postgr

I'm currently reading "Designing Data-Intensive Applications" by Martin Kleppman and it has an interesting chapter on database storage engines, including PostgreSQL (e.g. B-trees chapter). I would definitely recommend if you'd like a high level overview of how things work under the hood (it's a good mix of theory and practice). It also helps understanding why and when optimisations work.