tl;dr: "The Internals of PostgreSQL" by Hironobu Suzuki is a free online documentation (first published 2015) explaining how PostgreSQL's subsystems work internally, covering 12 chapters including query processing, concurrency control, VACUUM, buffer management, WAL, PITR, and streaming/logical replication. It targets PostgreSQL versions up to 19-beta and has been cited in academic papers, with a Chinese translation published in 2019. Recent additions include asynchronous I/O (June 2026) and a beta chapter on logical replication (May 2026).