In order keep configuration simple, please perform the following steps in Salesforce Classic (Booker25 is Lightning-Ready, you may afterwards use the app in Salesforce Lightning as you please)
When setting up a calendar configuration it is posible to show additional fields on the resource detail.
It is posible to show fields that are present on the resource object or on related objects.
Step-by-step guide
Click on Calendars in the menu bar
Click Go! next to All
Select the calendar for which you want to define the fields
Scroll down to the Titles related list
Click New Title to add new fields to display on the calendar resource detail.
Select Resource Title from the record type dropdown.
Specify the api field name of the field you want to display and select other options to tweak the appearance on the title
Field explanations for resource titles
Field name
description
Title Name
The API name of the field to display
Label
The label to display next to the field value
Bold
If the field value should be made bold or not.
Calendar
The calendar this title wil appear on.
Order
The order in which the titles wil be displayed lower values come first on the calendar.
Dynamic
If enabled the title is considered dynamic should be calculated based on the reservations in the current calendar view. For more information on dynamic titles see