How can I view cancelled appointments in the appointment book?

You can view cancelled appointments within the appointment book by following the below steps: 

Select your name at the top right-hand corner > 'User Settings'. See below:

Under the 'Appointment Book' tab select the checkbox beside 'show cancelled & deleted appointments'. See below:

Any appointments will then appear in black on the appointment book if they have been cancelled or deleted. 

Alternatively, you can run the 'Appointment Report' under the 'Reports' section > 'Appointment' > 'Appointment Report'. This will display all appointments. This report can then be filtered by 'appointment status to display only cancelled appointments. 

For further reading, see the below article:
How to see when an appointment was created