Estelle Renoit is a female Breton residing in the city of Chorrol. She is a merchant in books and owns Renoit's Books. You can only purchase the unique items Stormrider Scroll (resist shock 100 points for five seconds on self, shock damage 100 points) and the Marksman skill book Vernaccus and Bourlor from her.
Estelle was a key NPC during testing; she was used as a 'host' character in a trailer, displaying the Radiant AI's abilities. Her AI has since been dumbed down, however.