You also have ExUnit Official Documentation to start with.
I do not know a book dedicated to TDD in Elixir, but testing is not an issue for me, it’s more like a pleasure after using RSpec/Rails. It’s really damn fast. You don’t even have time to get a coffee while running the full tests 
But You need to test different things, like macro testing, OTP testing (You can assert that a process has/has not being called).
Others resources You might find useful.
- There is a dedicated testing chapter in Mastering Elixir (Self-published / Packt)
- It covers all sort of scenario… mocks/or not, channels testing, macro testing etc.
- If You want to test with Phoenix, You have Programming Phoenix (Pragprog) which includes tests.
- The same goes for GraphQL and it’s related book Craft GraphQL APIs in Elixir with Absinthe (Pragprog).
You might ping @peerreynders for a wise advice on the subject.
