[dguaraglia]

If what you are looking is a general book (not necessarily just for Postgres) I can't recommend the "Use The Index, Luke"[1] tutorial, and the accompanying book "SQL Performance Explained" by Markus Winand, enough.

It doesn't go into a lot of relational theory, but it does provide a somewhat low-level description on how indices and database data are stored, and why you want to use different kind of indices in different scenarios.

[1] http://use-the-index-luke.com

[hyperpape]

It's useful (probably more useful for most of us than any kind of deep dive into internals, at least for a first read), but I don't remember it going that deep into implementation decisions.