I’m reading Programming Phoenix 1.4 and to be honest wrapping my head around config files etc is really hard even if I have used Phoenix before. The book tells where the Endpoint is configured which is config :hello, HelloWeb.Endpoint, in config.exs. Then it jumps to the endpoint.ex.
I know what it’s doing, but there is a lot of more code. I can’t wrap my head around it completely.
I think this section needs more explanation, because we are not as proficient as the book authors. 
You don’t need to wrap your head around every bit of this code at this point. Try to digest as much as the book teaches you.
Hope José doesn’t mind but I’m quoting his post here from another thread since the topic gets brought up quite a lot:
So the advice for now is to get Programming Phoenix LiveView 
So the Programming Phoenix book is deprecated?
Perhaps the folks at PragProg should add a notice on the book’s web page, or lower the price or something. I don’t think it is in good taste to charge ~$30 USD for a book that has been unusable for beginners since Phoenix 1.7 was released almost 2 years ago.
The above is just my words and the book says on the tin what version it was originally written for (edit: and I just noticed it is almost 5 years old!).
My comment was mostly about a follow up edition: a new edition would effectively be a new book, so I’d rather direct folks towards Programming LiveView.
In any case, I will reach out to the other authors and we will notify PragProg accordingly.
I wasn’t trying to imply any malicious intent. 
I know of at least one person who recently bought the book and didn’t know that they would have to downgrade Phoenix in order to be able to follow the instructions. That’s certainly not an elegant footgun for newcomers to stumble into, so I’m happy to nudge things in the right direction.
The book has served its purpose, and while I think it would be nice to have “The Phoenix Book”, the docs are an excellent up-to-date resource, and have an unbeatable price.
I wouldn’t say it’s deprecated, but as José mentioned it is now several years old and targets Phoenix 1.4 - so if you want a book that covers more recent versions of Phoenix with all its latest goodies, such as LiveView (which most people probably will) then Programming Phoenix LiveView includes more of that recent stuff… until perhaps we see the next edition of Programming Phoenix.
PP 1.4 is also still worth getting if the topics it covers interest you. Personally, while I did not get around to finishing the book I really liked it - it basically shows you how to create a web framework, does an amazing job at selling Phoenix and includes loads of other interesting info too - such as about timing attacks! But despite all this, as you pointed out, most people coming to Phoenix will want a book that is more up-to-date, and right now, that’s Programming Phoenix LiveView (PragProg).