It's not a book, but Christophe Pettus' blog (https://thebuild.com/blog) has a lot of really good information. In particular, his talk "Breaking PostgreSQL at Scale" goes through the problems you run into as you hit different levels of scale (https://thebuild.com/presentations/2019-fosdem-broken.pdf)

Thanks! Very helpful, that talk looks like a (very) condensed version of what I was looking for.

It looks like the video for that talk is here: https://www.youtube.com/watch?v=XUkTUMZRBE8

FYI, for anyone interested Pettus does consulting work as the founder of https://pgexperts.com/. Highly recommend working with him if you need a postgres DBA.

Thanks for that! Been doing a lot of Postgres work but first time seeing that slide deck.