José Valim (the Elixir creator) is soon releasing a free ebook [0] on Ecto 2.0, and it's probably gonna give some good insights into Ecto in general, so I would definitely recommend getting that :)
You won't find anything better than hexdocs and stacked overflow at this moment (not in my experience at least). You don't get the dynamic method calls of find_all or the find(:hash key) magic but once you master Ecto.Query and all the different parts that make up Ecto you'll have much more flexibility and performance out of the box. The information is hard to find on Google but the Ecto source code and official docs are much easier to parse than ActiveRecord.
Somehow I've never heard of "sectors" and google isn't any help.
Is there a different name for this concept used in Elixir/Erlang world?