Please create a new field in the Customer module that allows a user to assign a sales tax code (created in the Sales Tax module) to each customer (according to their location) that will then auto-populate the sales tax rate when an invoice is created for a specific customer. This is important when a user has to abide by destination-based sales tax requirements. Otherwise the sales tax code and rate has to be applied manually each time an invoice is created.

For those users who don't need to abide by a destination-based sales tax requirement, they can simply leave the customer sales tax code field blank and Books would instead use whatever default sales tax was setup in the Sales Tax module.