json select question

All;

I'm working with a client running postgres 9.2, they have a table with a "json_data_string" column of type json

the data looks something like this with lots of rows for each (i.e. lots of json_data_string->book_name rows, lots of json_data_string->catalog_name rows, etc:

'{ "book_name": "Book the Fourth", "author": { "first_name": "Charles", "last_name": "Davis" } }'
'{ "catalog_name": "Catalog the Fourth", "author": { "first_name": "Charles", "last_name": "Davis" } }'
'{ "magazine_name": "mag4", "author": { "first_name": "Charles", "last_name": "Davis" } }'
'{ "category_name": "region", "author": { "first_name": "Charles", "last_name": "Davis" } }'


How can i pull a unique list of all json column names? such as book_name, catalog_name, etc

Thanks in advance