As I mentioned before, the books of evil excel in item battles because targeting the gremlin towers forces a damage spell and the mage heavy troops usually have a hard time to kill the books.
And actually, while upgrading the arch mage staff in my current harder game, I had the pleasure to experience the most powerfull book of evil I've seen so far with a level 17 mage. I'm not sure if spell powers are increased in such item battles, but one of my three books of evil got a lightning spell off which caused about 25.000 damage in total. I checked the numbers, it's not a joke. This specific book lived long enough to cast about 4 - 5 spells. Two of them were lightning spells with tremendous damage. In the whole battle it caused about 50.000 damage which is more than my entire army together was able to cause. I would definitely have lost the battle without the books.
Therefore, while the book of evil surely isn't the best choice in every situation, it's most certainly the spell with one of the highest power potentials in the game. I got this result with Int 25. Now I have Int 30 and +20% lightning damage through the arch mage staff. It'll be interesting to see how powerfull the book can get...
