Because there are many different implementations, with different trade-offs and design decisions. Also, the implementation-specific documentation is typically kept fairly close to the code, as the docs need to be kept in sync.

So for example if you need to know how indexes in PostgreSQL work, look into the READMEs in the proper directory:

b-tree indexes: https://github.com/postgres/postgres/blob/master/src/backend...

gin: https://github.com/postgres/postgres/blob/master/src/backend...

brin: https://github.com/postgres/postgres/blob/master/src/backend...

etc. The READMEs also include links to related papers etc.

This is a good summary paper on the database internals and major architectural decisions: http://db.cs.berkeley.edu/papers/fntdb07-architecture.pdf

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