Having this spellbook (or the prospect of Cthulhu obtaining this spellbook) creates forced behaviors in your opponents' actions. They will be reluctant to build gates in the ocean areas.
Use this spellbook as a bargaining chip with your opponents. "I will do X if you build your gate in the ocean."
The longer the game lasts, the more power this spellbook will generate. Not only will you see the power return round after round, ocean areas will begin to fill with gates as real estate becomes more limited.