While looking for references and/or resources to support my research on the ‘new’ Office Add-ins I repeatedly stumbled over the sample chapters of one book in particular:
Building Office Add-ins using Office.js
https://leanpub.com/buildingofficeaddins
The book is published on LeanPub and this means that it is work in progress. The author is continuously working on the manuscript and is potentially never finished. Even the pricing is flexible, the author specified a minimum and a recommended fee. You can decide if you want to pay the minimum, recommended or even a higher amount for the book. At this time, the minimum is set to $15.99 and the suggested price is set to $19.99 but as said you can up the price to $500 if you like 
You don’t have to worry the information provided is legit, the book is written by Michael Zlatkovsky who works at the Office Extensibility Platform team at Microsoft. The information is therefor right from the source, even better … Michael has been a key participant in the design of the new Office 2016 wave of Office.js APIs:
https://leanpub.com/u/Zlatkovsky
Michael and I go back quite some time already, I met Michael years ago, I thinks around 2012, when we were all still developing VSTO Add-ins. Michael always was very helpful and supporting when I needed information or struggled with issues.
Now get on over to LeanPub site and buy the book!
Did I mention already where to get it?
https://leanpub.com/buildingofficeaddins
I’m sure it will help you and me in learning the details of developing Office Add-ins using Office.js