My quick response: no. 
All the examples on the book will work and you can still organize your code the same way you do on 1.2 and use 1.3, it’s totally backward compatible.
But as the book have examples that explains why things are where they are when you create a new project, the readers could be a little bit confused if they follow the book examples. I would wait for Programming Phoenix 1.3, or would not update to 1.3 yet.
Fun fact: You can’t exist in the innovators/early adopters phase of a product’s adoption lifecycle and expect to have the same level of tooling and comprehensive educational material coverage that is typical for products in their late majority/laggards stage.
To the point: 1.3 is still just a “release candidate” - “It’s done, when its done”. If you have an urgent need now, stick with the stable version (of the product or the book). If this is about “I can’t be bothered learning the old stuff” then figure it out from the existing documentation or be patient - and if Phoenix continues to be successful then even 1.3 isn’t the be-all and end-all anyway; it’s simply a milestone in its continual evolution.
Side-note: Everybody want’s to be on greenfield projects/products because there is no technical debt/legacy code to contend with. But if there is an opportunity to participate on an existing (brownfield) Phoenix project, you’ll need to know the “old way/stuff” anyway in order to be able to find the issues that need to be nudged (refactored) towards more up-to-date practices.
It’s funny I was googling to see if there any books that deal with Phoenix 1.3 as it is stable release right now or just wait until book get released but I found this thread and found your response.
I guess I can work and learn from Phoenix v1.2 tutorials/books/courses first then try and migrate the project from 1.2 to 1.3 as part of learning process.
