Recently we bolstered our Drupal 8 Module Development Guide with a few new tutorials. In particular I'd like to highlight 5 of them today that all have to do with Drupal's Field API from a developer perspective. The Field module has both a user interface (UI) for site builders to use when constructing entities, and an API, which developers can use to programmatically interact with and extend the functionality that core and other contributed modules provide.
The new tutorials about Field API are:
The overview tutorial walks through the main components and functionality that the Field subsystem provides. From there we take a closer look at each of the three main components: field types, widgets, and formatters. These tutorials include a broad view of core implementations as well as creating your own fields with custom code. Lastly we look at the main hooks provided by the Field API that can be used to interact with fields provided by either core or other contributed modules.
Hopefully you find these to be a useful jumpstart on your way to understanding how Drupal 8's Field API works. If there are other particular aspects of the field system you're curious about, we'd love to hear about them.