A butler book is a tool often used by those in service oriented positions to help organize information.
The book is intended to help a person in service to another keep organized and improve day to day service by acting as a compilation of every unique detail about the person being served.
Varieties of butler books might include small pocket sized notebooks (possibly worn around the neck on a string), a binder, or the increasingly popular PDD/Personal Digital Device.
This may include certain things such as the d-type's work schedule, what day the trash has to be put out, what their favorite flavor of tea is, what temperature they like the room at, lists of specific tasks, daily rituals, alarm codes and other passwords, details of medications, what foods they like in the house, wish list items, etc.
Essentially, any time the s-type gains a new fact that assists to service the d type, such as a preference or task, it should be recorded and put into the butler book.