I think this is the most up to date.

https://www.interdb.jp/pg/

Different author but also very interesting.

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.

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

https://github.com/wal-g/wal-g/issues/1385