Will my paying customers from Zoho Checkout be automatically added in Zoho Books as contacts?​

Yes, all your customer details and invoices that are recorded via Zoho Checkout will be auto-synced to Zoho Books.

Zoho Books invoice information Zoho Checkout invoice information

In case of customers, if you already had an existing Books customer with the same name, then you will have to manually merge the contacts to avoid duplication. To do so:

Merge Books contacts