Couldn't agree more on moving the SoM technology further down in the magic tech tree.
We have 24 techs in Civilization, 17 in Warfare and 20 in Magic (inculding tech that you can research indefinitly). We need 8 techs to reach the Book of mastery technology right now which is pretty silly.
Getting 8 techs in Civilization (up to cooperation, mining, maybe education and forgetting agriculture), then 8 tech in warfare (up to blacksmithing and maybe longbows) and diving to the Book of mastery tech you can lock out an insanely quick SoM victory just sitting on a few cities (3-5) up and including expert difficulty (tried this out in ridiculous recently but no success so far as the AI just has too many stacks).
My 2 cents:
Put Arcane Apparel where Book of Mastery stands: There is no reason why we should be able too research Arcane Apparel without any knowledge in Arcane Mastery. Therefore, "Even those who lacks inherent arcane abilities can use its power" Arcane Apparel descrpition would be justified since our mages can craft arcane apparel since they have acquired beforehand knowledge of arcane magic.
Book of mastery: Requires The Black Quire and Magical Forging. No reason why we should be able to craft arcane apparel without knowledge, no reason we should be able to build the Forge of the Overlord without any Magical Forging knowledge. Book of mastery would lead to Arcane weapons and Arcane Armor, the most powerful pieces of armor your faction can acquire.
Tower of the witch moved to either Sorcery or Arcane Mastery, I'd say Sorcery since it matches the description and the Ereog's Tower is already at Arcane Mastery.
Tower of the Padars moved to Arcane Mastery or Immortal Codex, would match the description of the Tower.
Tower of the Swords moved to Magical Forging, would match the description of the Tower.
Each Tower could have a specific additional cost to be built like mana for TotW or Metal for TotS.
Move Call of the Titans somewhere else, likely to Sion Temple or to the new Arcane Apparel.
Last but not least, the SoM in itself cost feels VERY controversial. So for 250 mana I can bend all the Magic of Elemental to my will, but to curse or bless a city it costs me twice as much power? Really? A bit of roleplay for a second here please. There is no way making the sun shine brighter on a city cost me twice as much magical resource as channeling all magical energies in the world.
Adjust mana costs for spells in the Magic tech tree: Curgen's Volcano should almost match the cost of the SoM or be the same (800? 1000?).
Thank you and good gaming.