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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.

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.

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