Obtaining printed copies of the PostgreSQL manual - Mailing list pgsql-general

Hi Postgres fans,

I would like to obtain a printed copy of the PostgreSQL manual. It
appears that the manual is available on Amazon at the link below, split
into six volumes. 

    https://www.amazon.com/dp/B0CQTMLZ5Y?binding=paperback

My question is: Are these copies legit and actually made available by
the PostgreSQL project? It appears so, but I'd like ensure my purchase
goes toward supporting the project in some small way. 

Also, is anyone aware whether these printed manuals are available from
any other booksellers online?

Thanks,

- Ben