How works Necro Call?

[ArkhanTheBlack]

Nice but I'm surprised that a book isn't killed in one turn when exposed like that

A book of evil from a level 3 spell has a toughness which is comparable with a giant or cyclops. Against spells and magic damage, they are pretty much invincible.

and that you never get hit just by moving back of one field.

If your own units stay back and you target a ranged enemy stack at the other side of the battle field, there's almost no chance that you get hit. If all enemy units run forward and yours too, you probably only have one try at the start of the battle. It's simply situation dependent, and if you only target distant enemies you can't get hit too hard. Sometimes I've no problem to get by 100 - 200 damage if the opponent gets several thousand damage in exchange.

Is this in impossible mode?

It's in hard mode. Though the increased stack size are not really a big problem. If two summoned books cast lightning, and the enemy then sufferd about 10.000 damage in total, that can even reduce bigger stacks to a size that they can't kill a book anymore.
The problem is if the books use spells like sleep or reduced defense. That's just not enough for 30 mana. And on hard it seems more difficult to get real 'killer books'. In short words: The book sucks with low int characters and is great with high int characters. Though I can't give a precise min INT value to get good books or bad books since it seems to differ between the various difficulty levels.