This is something I have on the list to clarify in a near future version. The lightning books increase the cooldown by 5x and the cost by 3x. So equipping only one of them, either one, increases the cost to 30. Equipping both goes to 90, but you're looking at chain lightning with a 50% chance to stun each of them. Might be a bit too high (I'm going to drop that to 2x instead of 3x for the stun one, so it'll only be 60 together), but it's definitely a more powerful spell.