Mickey Petersen’s excellent Mastering Emacs has a new update covering Emacs 26 & 27. If you already have a copy of Mastering Emacs, you’ve probably received an email telling you how to get the update (for free). If you don’t yet have a copy and are an Emacs user, I can’t recommend it enough.
Mickey has long been famous—at least in Emacs circles—for his Mastering Emacs blog that covers the deeper aspects of Emacs in detail and helps Emacs users get the most from their editor. The book is written in the same style but is not merely a collection of old blog posts. No serious Emacser should be without it.