To a certain degree that makes sense. It is targeted towards programmers who are already conversant in the higher level concepts that are typically found in most programming languages and Programming Elixir shows how to implement those concepts in Elixir, perhaps making some allowance for the reader not being familiar with the functional style.
More basic resources with a narrower scope are Introducing Elixir 2e together with Études for Elixir.
Whoa - hold your horses and slow down a bit.
While cobbling together an HTML page is pretty basic - building a web application requires the convergence of multiple technologies to put those pages inside the browser. A typical basic Phoenix project involves**:
- Core Phoenix which of course uses Elixir but in itself is an OTP application (a layer of abstraction on top of Elixir). To some extent OTP can be ignored to get started but ultimately OTP is necessary to play to Phoenix's strengths.
EEx for server-side rendered templates.
In fact Elixir in Action uses Plug directly in Chapter 11.3 Building a Web Server to stay focused on Elixir and not get distracted by "all that other stuff".
** (Not to mention Ecto and PostgreSQL)