A brief response of the “Thank you for taking time to send me this update. Happy reading,” variety is a lovely way to acknowledge receipt of her email. After that, let it be for a while. The volume of emails that an agent gets in a day is large, and I’d err on the side of not adding to it unnecessarily. You’ve established a gentle, professional rapport with the email exchange; leave it at that. Later, when she sends another email telling you how much she loved the entire book, you can respond to her chattiness in kind.