The butler's diary is found in the Elona+ mod of Elona, added in version 1.83. It can be randomly generated in dungeons, or bought from rich person Stoke for 4500 music tickets.
Using the book will display the text, "A butler falls from the sky." After this, a butler will join you. If your party is full when you read it, the book will still be consumed.
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The book is located at 429 (Row 13, Column 0) by the character sprite zero-based numbering system. It is tinted randomly like a normal spellbook.