Solomon's Grimoire is a powerful book of djinn magic that once belonged to the great magus Solomon. A Grimoire is a sorcerer's manual for binding djinn. The Solomon's Grimoire was an important book to the djinn and whoever possessed it gave him/her immense power.
Only the Blue Djinn of Babylon owns a copy of the book and keeps it in the Heisber Electron safe in Berlin. Any attempt to discover the combination will change the lock which makes it nearly impossible to break. The only person that can access it is the Blue Djinn herself. In the second book it is supposedly stolen by a book thief, Izaak Balayaga , which causes the twins and Nimrod to search out Izaak, worrying that it could upset the balance of power. Later, they learn it is an elaborate plot by Ayesha Godwin to persuade Philippa to become the next Blue Djinn of Babylon.
When Solomon was Israel's king, Solomon wrote the book containing all his wisdom and described how power can be obtained on djinn, angels, and demons of the earth. Asmodeus tried to steal his book that he wrote in an act of revenge. The demon king had access to library and found the grimoire. Fortunately, the next person to receive the book was Ishtar who passed it on for generations.