Welcome to the Seller Community @drkaseyhovest! There isn't a way to send a customize email to a client who has booked a specific Appointments with you. I'll send that request to our Appointments team. 

In the meantime you could consider using Square Marketing. I'm not sure if this will work for your practice but with this feature after client has booked an Appointment you can assign them to a group from in your Customer Directory (e.g., 'New Patients'). Then you can send an email to that group, in your case you could include links to documents or patient reminders in that message. 

Hope this is useful! If you have any other questions don't hesitate to reply here!