The latest book version, B 13.0 supposes to use the latest Phoenix 1.8.0 heex template variable interpolation syntax, what is not the case, unfortunately.
On page 26 we still can see:
<h2>
<%= @message %>
It's <%= time() %>
</h2>
The correct syntax for the above code snippet would be:
<h2>
{@message} It's {time()}
</h2>