The book implies that the code samples should be typed in while working through the book. However sometimes the author leaves you to complete some of the code on your own (queries). It would be useful if this were more explicit.
In other places the iex sessions will fail due to undefined variables (path to the stored files is a prime example).
There is also a suggestion to use custom code to solve a serialisation problem, but no mention is made of where to get it (I assume it’s in the download for the book).