After you find the Tome you need to surround the demon with runes. The commissioner (in the back office) will help you with that.
This is the low-spoiler guide for "Helping the commissioner" puzzle in The Darkside Detective - A Fumble in the Dark. Unlike in normal walkthrough, you get gradual hints on how to solve the puzzle. Click the big button below to see your first hint. If the hint contains a spoiler, you will be warned about it before clicking.