Code for step 5

lib/book_store/application.ex

@@ -15,6 +15,8 @@ defmodule BookStore.Application do
       {Phoenix.PubSub, name: BookStore.PubSub},
       # Start the Book Store Registry
       BookStore.BookRegistry.child_spec(),
+      # Start the Book DynamicSupervisor
+      BookStore.BookDynamicSupervisor,
       # Start the Endpoint (http/https)
       BookStoreWeb.Endpoint
       # Start a worker by calling: BookStore.Worker.start_link(arg)

lib/book_store/book_dynamic_supervisor.ex

@@ -0,0 +1,32 @@
+defmodule BookStore.BookDynamicSupervisor do
+  use DynamicSupervisor
+
+  alias BookStore.Books.{Book, BookProcess}
+
+  def start_link(opts) do
+    DynamicSupervisor.start_link(__MODULE__, opts, name: __MODULE__)
+  end
+
+  @impl true
+  def init(_opts) do
+    DynamicSupervisor.init(strategy: :one_for_one)
+  end
+
+  def add_book_to_supervisor(%Book{} = book) do
+    child_spec = %{
+      id: BookProcess,
+      start: {BookProcess, :start_link, [book]},
+      restart: :transient
+    }
+
+    {:ok, _pid} = DynamicSupervisor.start_child(__MODULE__, child_spec)
+  end
+
+  def all_book_pids do
+    __MODULE__
+    |> DynamicSupervisor.which_children()
+    |> Enum.reduce([], fn {_, book_pid, _, _}, acc ->
+      [book_pid | acc]
+    end)
+  end
+end