As your template shows, within a loop pandoc makes a local variable with the same name as the array ('author' in your case). So to iterate through the inner list, simply use the same 'for' mechanism on the inner variable.
Thus, you should use
TEMPLATE
$for(author)$
$for(author)$
$author$
$endfor$
$endfor
You can also use $sep$ to specify a separator to use between the elements of the list.
Note that if the inner list has elements with different meanings (rather than just a list) then you should use a list of dictionaries.
YAML
Author:
- {name: Iain Banks, book: The Algebraist}
- {name: Isaac Asimov, book: Foundation}
TEMPLATE
$for(author)$
$author.name$ wrote $author.book$
$endfor$
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