Quote:
"By making a reservation through Booking.com, you enter into a direct (legally binding) contractual relationship with the Supplier with which you make a reservation or purchase a product or service (as applicable). From the point at which you make your reservation, we act solely as an intermediary between you and the Supplier, transmitting the details of your reservation to the relevant Supplier(s) and sending you a confirmation email for and on behalf of the Supplier"
Source:
https://www.booking.com/content/terms.en-gb.html
So your contract is entirely with the owner of the accommodation. (However still make sure that you take up the opportunity to submit a review to the Booking.com website).