Greater Storms Spellbook. White or Black yes. You'll wait a long time in Deli getting White, but you can get it at any point in the game. Black requires you to kill Lost, which is a somewhat hard point to get to if you don't have good weapons. I would suggest getting Black if you don't want to spend hours to get White, but if you don't have good enough weapons to kill Lost and Democan, get White.
The Regular, Purple, and Green are not worth the cash due to their damage. Storms is much better if you main in magic and don't care to get a quite a bit closer as its DPS will be higher than the lower tier Salamanders.
*Btw, if you want help with Tower I would be willing to help you. I have F40s and a want to improve in the game, so I' willing to do Tower runs. Just a thought.