Good overview of programming IN Postgres?

Is there a good overview of programming WITHIN Postgres? Not writing code in client languages that communicates WITH Postgres, but how to take full advantage of Postgres’ unique programming features — the advanced type system, the various PL languages, triggers, stored procedures, functions, …

I’m thinking both how all those things work, but also how to think about effective architectures given that design palette.

The Postgres documentation is great and is the ultimate reference, but is there a good overview for someone who wants to take best advantage of all these features?

Thinking if there isn’t such a thing that I might write it. But I don’t want to duplicate effort.