Once you installed the Zoho Books app in your Freshdesk helpdesk, Zoho Books Integration section will be automatically displayed in the far right pane of every ticket created in Freshdesk. 

If the Contact who submitted the ticket is registered in your Zoho Books account with the same email address, then you will be able to view this Contact details. Click on the 'Open Details' button to see the Customer Details tab.

You can select what Zoho Books fields should be displayed in your Freshdesk ticket in the app settings.