Since the only way to know when an invoice email has been opened in Zoho Books is by going into the application and hovering over the mail icon in the Invoices section on Zoho Books, my other users never know when it has been viewed.  

I've played with the workflow rules but I can't find any based on signals or email opened so I'm assuming I will need to create a custom function for this? Email notification would probably be easier I'd think but a Cliq notification would also work.

I have never written a Books custom function so any guidance would be appreciated or a template.  I do have Zoho Finance integration with CRM so would doing it through the Books app be the right way to go or should it be in CRM?