Geoffrey Lessel’s 2019 book, Phoenix in Action, was written for Phoenix 1.4.
I found that the book’s code examples did not match the current Phoenix 1.7 web application structure.
Reverting to Phoenix 1.6, I was able to build, run, and test the example Auction Web application.

As I worked through the book, I made the following changes to accomodate Phoenix 1.6:

6.1 Installing Phoenix on your system

Install the latest version of Phoenix 1.6.

mix archive.install hex phx_new 1.6.16

6.2 Creating a new Phoenix application

Phoenix 1.6 applications include a mailer by default.
The auction project does not need it.
When generating the auction_web application, omit the mailer with this command.

mix phx.new.web auction_web --no-ecto --no-mailer

Listing 6.3 shows adding this line to configure Phoenix to use Jason.

config :phoenix, :json_library, Jason

Phoenix 1.6 generates that line automatically; there is no need to add it.

6.2.1 Running your server for the first time

The “good news” also includes this advice:

Your web app requires a PubSub server to be running.
The PubSub server is typically defined in a `mix phx.new.ecto` app.
If you don't plan to define an Ecto app, you must explicitly start
the PubSub in your supervision tree as:

    {Phoenix.PubSub, name: AuctionWeb.PubSub}

That line should be added to the list of children in
auction_umbrella/apps/auction_web/lib/auction_web/application.ex:

  @impl true
  def start(_type, _args) do
    children = [
      {Phoenix.PubSub, name: AuctionWeb.PubSub},
      # Start the Telemetry supervisor
      AuctionWeb.Telemetry,
      # Start the Endpoint (http/https)
      AuctionWeb.Endpoint
      # Start a worker by calling: AuctionWeb.Worker.start_link(arg)
      # {AuctionWeb.Worker, arg}
    ]

Running the web server without this line results in a barrage of log messages such as:

08:55:41.210 [error] GenServer #PID<0.629.0> terminating
** (ArgumentError) unknown registry: AuctionWeb.PubSub
...

When first run, phx.server complained:

>mix phx.server
warning: the :gettext compiler is no longer required in your mix.exs.

Please find the following line in your mix.exs and remove the :gettext entry:

    compilers: [..., :gettext, ...] ++ Mix.compilers(),

I deleted :gettext from this line in auction_umbrella\apps\auction_web\mix.exs:

    compilers: [:gettext] ++ Mix.compilers(),

10.4 Adding site navigation

In Phoenix 1.6, root.html.hexx contains the HTML boilerplate, including the <header>.
That file has a placeholder, @inner_content, for the contents of app.html.heex,
which provides two alerts and references additional @inner_content.
The site navigation can be added before those alerts.

11.3.3 Adding the list of bids to the view template

In Listing 11.21, item_bid_path should be Routes.item_bid_path in this line:

        <%= form_for @bid, item_bid_path(@conn, :create, @item), fn f -> %>

12.2 Connecting a user to a channel and a topic

The Phoenix app was missing user_socket.ex. Create it with these commands:

>cd auction_umbrella\apps\auction_web
>mix phx.gen.socket User

Follow the resulting instructions to add the socket handler to your lib/auction_web/endpoint.ex, for example:

    socket "/socket", AuctionWeb.UserSocket,
      websocket: true,
      longpoll: false

12.2.2 Getting the user’s browser to join a topic

With Phoenix 1.6, the socket.js file is named user_socket.js.

Listing 12.4 shows the automatically generated channel topic:subtopic.
Phoenix 1.6 instead generates the channel room:42.

In Listing 12.6, also uncomment this line near the beginning of the file:

// import "./user_socket.js"

12.4.1 Refactoring the rendering of an item’s bids

The code in Listing 12.12 must be saved in bid.html.eex, not bid.html.heex.

Phoenix 16 introduced a new HTML engine that uses HTML-aware template files
(HEEx: HTML Embedded Elixir).
I did not initially notice that many automatically generated templates had the
.heex extension.

See Phoenix 1.6.0-rc.0 released!
for a description of HEEx templates.

See Phoenix.View.render_many can't render .leex templates · Issue #3952 · phoenixframework/phoenix · GitHub
for a discussion of render_many and HEEx files.

14.4.3 Viewing documentation with ExDoc

Add a project name to auction_umbrella/mix.exs:

  def project do
    [
      name: "Auction Umbrella",
      apps_path: "apps",
      version: "0.1.0",
      start_permanent: Mix.env() == :prod,
      deps: deps()
    ]
  end

14.4.4 Adding examples and doctests

In the first example in Listing 14.22, start the second line with ...> and correct the double quotes
around the password confirmation value.

    iex> insert_user(%{username: "geo", password: "example",
    ...> password_confirmation: "example", email_address: "test@example.com"})
    ...> result = get_user_by_username_and_password("geo", "example")
    ...> match?(%Auction.User{username: "geo"}, result)
    true

14.5 Writing tests For Phoenix

The test runner displayed this error instead of the error shown in Listing 14.25:

  1) test GET / (AuctionWeb.PageControllerTest)
     apps/auction_web/test/auction_web/controllers/page_controller_test.exs:4
     ** (ArgumentError) cookie store expects conn.secret_key_base to be set
     code: conn = get(conn, "/")
     stacktrace: ...

Fix by adding a secret_key_base to the test.exs auction web endpoint:

config :auction_web, AuctionWeb.Endpoint,
  secret_key_base: "Dzlf6LT5j14IS16DQHviw2OuMiNjHBGGkrCDe8fMbR41Us/RMCPQfWzkigWa7Ags"

Next, the test runner displayed this error instead of that shown in Listing 14.25:

  1) test GET / (AuctionWeb.PageControllerTest)
     apps/auction_web/test/auction_web/controllers/page_controller_test.exs:4
     ** (DBConnection.OwnershipError) cannot find ownership process for #PID<0.627.0>.

     When using ownership, you must manage connections in one
     of the four ways:

     * By explicitly checking out a connection
     * By explicitly allowing a spawned process
     * By running the pool in shared mode
     * By using :caller option with allowed process

     ...

Fix by modifying auction_web/test/support/conn_case.ex to
checkout Auction.Repo in a sandbox and set its mode to shared.
Replace the setup _tags do section with:

  setup tags do
    :ok = Ecto.Adapters.SQL.Sandbox.checkout(Auction.Repo)

    unless tags[:async] do
      Ecto.Adapters.SQL.Sandbox.mode(Auction.Repo, {:shared, self()})
    end

    {:ok, conn: Phoenix.ConnTest.build_conn()}
  end