Data-options not in object context

I'm attempting to create a custom content type (podcast) and then add fields from a custom plugin (biblref). In the podcast.yaml file, when I use:
'@data-options: '\Grav\Plugin\BiblerefPlugin::getBookOptions'
I get the following error:

Using $this when not in object context

In my bibleref.php file, I'm attempting the following:
public function getBookOptions() {
...
$book_list = __DIR__ . '/' . $this->config->get('plugins.bibleref.book_list');

How do I reference the configuration yaml properly?

Thanks in advance

I was originally putting the new field definition in my theme, but moving it out to the bibleref blueprints still produced the same error.

The getBookOptions() method needs to be static, and as such you can't use $this, you would have to use $static.

Thanks for the reply. I ended up moving my file to the DATA_DIR ahead of time and referenced it there.