If you're interested in a "good-practice" type introduction to use of Packer and Terraform together, you may be interested in a talk [1] (or the accompanying repo [2]) I gave at HashiDays New York last year.
Unfortunately I never finished getting everything I was aiming to done in that repo, but may come back to it and update it at some point...
Disclaimer: I used to work at HashiCorp on both Terraform and Packer.
[1]: https://www.youtube.com/watch?v=8ZRa0lLq8OU
[2]: https://github.com/jen20/hashidays-nyc
(edited to change "best-practice" to "good-practice", since it's more accurate...)