This book is still a work in progress, therefore you can freely read it and contribute through Github.
From my point of view, this book should be considered not more than a manual listing most of the features of ECMAscript 2015 with examples. It’s not yet finished, but imho i doubt that it will be more than a list of features.
On the other side, it can save you a lot of time if you’re looking for one place to get familiarized with most of the new constructions that were added to JS recently and you’re not in a mood to chase a lot of blog posts dispersed throughout the Internet.
My score (book is still 60% complete, hence this is not a final score) 3/5
