Drupal 8 Migration Resources
Drupal's official documentation for the Migrations and Migrate API
The guide to the UI is still just a stub: https://www.drupal.org/docs/8/migrating-to-drupal
The Migrate API documentation is pretty comprehensive, and includes ways to use it: https://www.drupal.org/docs/8/api/migrate-api/
Building a custom migration in Drupal 8 blog series by Tess Flynn (socketwench)
Migrate All the Things presentation by Dave Vasilevsky
Migrating SQL to Drupal 8 with Migrate Tools and Migrate Plus
Avoid sending emails while doing a migration on Drupal 8 by David Valdez
Slides from Mauricio Dinarte and Benjamin Melançon's presentation at DrupalCamp Twin Cities 2017:
Extra tips, not in the slide deck:
* Use --limit while working on a large migration.
* Use git! Commit all changes to the migrate_plus.migration*.yml files you're working on. Then you can export configuration (such as for changes made to your field definitions) without fear, and git checkout your migrate_plus.migration*.yml file or files, and then import it all.