Each time you approve or decline an individual request, the member receives an email. You can create a custom default message that will always be sent, or you can change the approval and decline emails for a single book.
To change the default email, first login to your account. From your Dashboard, click "Edit Approve and Decline Emails" under Requests. Edit the text, and press the Update button to save. 
Take a look at some template messages:
Templates for Approval emails
Templates for Decline emails