This should work:
function wpse_20160421_get_author_meta($object, $field_name, $request) {
$user_data = get_userdata($object['author']); // get user data from author ID.
$array_data = (array)($user_data->data); // object to array conversion.
$array_data['first_name'] = get_user_meta($object['author'], 'first_name', true);
$array_data['last_name'] = get_user_meta($object['author'], 'last_name', true);
// prevent user enumeration.
unset($array_data['user_login']);
unset($array_data['user_pass']);
unset($array_data['user_activation_key']);
return array_filter($array_data);
}
function wpse_20160421_register_author_meta_rest_field() {
register_rest_field('post', 'author_meta', array(
'get_callback' => 'wpse_20160421_get_author_meta',
'update_callback' => null,
'schema' => null,
));
}
add_action('rest_api_init', 'wpse_20160421_register_author_meta_rest_field');