Utilise the science of mixology, do you notice any similarities between the letters on the cocktail menu and those in the customer feedback book?
Helpfulness, Cleanliness and Overall (HCO) - each review will create a chemical formula e.g. H2C5O1. These will need to be converted into letters next, as long as you've found the other book UNDERNEATH.
If you haven't already, move the customer feedback book to the side of the screen to reveal the chemical formula key.