Check out “Drupal 8 Theming with Twig” by Chaz Chumley over at Packt. It’s a straightforward intro to using Twig with Drupal 8. Between now and Jan 1 2017, all e-books are $5!

Note: I did run into an issue with using the db to update between chapters and Drupal 8.2.4. There are a few fields that aren’t there. Other than that, a good read.