Google recently began testing appointment bookings in the knowledge panel, through a Demandforce and Intuit partnership.  These bookings, called Place Actions Forms, are displayed within the organic knowledge panel in the Google search results.  Google is now expanding these appointment bookings to more providers, including the most popular booking appointment platform used by salons in the US.  They have also expanded them to mobile as well.

Here is how it displays in the mobile search results, both in its collapsed version, and when expanded.

And when expanded, also showing the information about the card.
When you click through, you see selection screens that allow you to select more detail about the service as well as the price.

This one is powered by StyleSeat.com.  Booker, another service booking tool, has also added support for bookings via Google knowledge panel.

This is still in beta, according to Booker, so it is likely we will see even more service providers added.  But while there is no charge by Google to include these bookings in the knowledge panel, the third parties providing the data to Google all seem to charge for the service.

It also seems to be only testing within the US at this time.

Thank you to Kristine Schachinger for the screenshots.