PostgreSQL Internals [pdf] (2023)

To your point, PG15 has a pretty meaningful change to its base backup protocol

They keep publishing Russian version of this book, with the latest covering PostgreSQL 18: https://edu.postgrespro.ru/postgresql_internals-18.pdf

AI should do a decent job translating it to English.

On an unrelated note, this reminds me of the excellent books published by Mir Publications. Russian books in general are of very high quality.

I think this is the most up to date.

Different author but also very interesting.

This is probably the best and most beginner-friendly resource.