Dear Wendy…
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.
Happy writing!
The Editor