I also recommend his book, Mastering Emacs [2]. I learned so much from that book. It's great for both beginners and medium/long term users.
I'm mostly an autodidact so I rarely buy technical books, but I wanted to support his efforts based on reading his blog before it was published.
[1] https://www.masteringemacs.org/article/whats-new-in-emacs-25...
[2] https://masteringemacs.org/book