If you want to create a custom email for a particular title, press the "Edit Approval" or "Edit Decline Email" link from the Manage Requests page of the individual book:
OR from the Title Details Page, under the "Requests" section:
You can also create a DEFAULT Approval or Decline message, which will become the default used for all future approvals or declines except for titles with customized messages.
Take a look at some sample messages:
Templates for APPROVAL emails
Templates for DECLINE emails