The 2nd edition of Deep Learning with Python[1] (by François Chollet of Keras fame) has a pretty good chapter on the transformer architecture[2]. In fact, the explanations in the book are clear and lucid across the board.

[1]: https://www.amazon.com/Deep-Learning-Python-Francois-Chollet...

[2]: https://en.wikipedia.org/wiki/Transformer_(machine_learning_...