In order to do that, you need to go to the Settings page, on General tab and set Show booked time slots to enabled if you want to display booked time slots.
Note that these time slots stay unavailable for booking.
You can also edit the text of the Book button and insert placeholders.
For example: ‘Booked by #username’.
To do this way, go to the Settings page, Translation tab and edit the Book button text (time slot).