This book is a tutorial. It aims to help you gradually learn the language. But once you’re familiar with the basics, you’ll need other resources.
But being that formalized, it’s difficult to understand at first. So if you need the most trustworthy source of information about the language details, the specification is the right place. But it’s not for everyday use.
A new specification version is released every year. Between these releases, the latest specification draft is at https://tc39.es/ecma262/.
To read about new bleeding-edge features, including those that are “almost standard” (so-called “stage 3”), see proposals at https://github.com/tc39/proposals.
Also, if you’re developing for the browser, then there are other specifications covered in the second part of the tutorial.
Although, it’s often best to use an internet search instead. Just use “MDN [term]” in the query, e.g. https://google.com/search?q=MDN+parseInt to search for the parseInt function.
To see their support among browser-based and other engines, see:
https://caniuse.com – per-feature tables of support, e.g. to see which engines support modern cryptography functions: https://caniuse.com/#feat=cryptography.
https://kangax.github.io/compat-table – a table with language features and engines that support those or don’t support.
All these resources are useful in real-life development, as they contain valuable information about language details, their support, etc.
Please remember them (or this page) for the cases when you need in-depth information about a particular feature.