To add a "Book Now" button to your Facebook page, just click on "Add a Button":
Then, select the "Book Now" option and click on "Next":
Afterwards, select the "Link to Website" option:
After, you will be prompted to insert the link where your clients will be able to make bookings:
Input the url and click on "Save":
Click on "Finish", and it's done:
There you go... your "Book Now" button is ready.
To test it, just mouse over the "Book Now" button and you will be able to access the "Test Button" option:
When you click on "Test Button", if you are taken to the link where people can make their bookings, you did it.