Spellbooks are combinations of spell/creature pairs with five possible pairs to choose for each of the nine levels. Each of the gods has their own pairs and each pair can be that of a different god, making the number of possible spellbooks equal 5 to the power of 9, or 1,953,125 different combinations.
Standard spellbooks are unaltered spellbooks assigned along with a patron god.
The strategy with Persephone is to constantly heal and shield your creatures, assuring they outlive your enemy.
The strategy with Stratos is to benefit from your versatile early game creatures until you get Storm Giants which deal great damage to creatures around them. Use Statos' spells to back up his creatures, as they do a lot of damage.
Pyro excels at dealing damage. Both his creatures and his spells inflict a lot of it. Since his creatures aren't as durable as those of other gods, however, they rely on their ability to kill their enemies before they die themselves. Many of Pyro's creatures are quite mobile to make up for their lack of health.
The strategy with Pyro is to use his spells and creatures to destroy the enemy army before they destroys yours. Your army has firepower, both in fiery spells and dangerous ranged attackers. Your melee troops won't last long in a drawn out battle, but Firefists get stronger with each hit they take and your cogs deal area of effect damage when they die. So long as you can keep the pressure on then the fires of Pyro can cook any wizard and out-damage any opponent.
Collecting souls may be more important to you than to anyone else, since your army will be more likely to suffer casualties. Everything about Pyro hurts the enemy, and your shield spell can allow you to charge into the fray to collect souls, wounding melee attackers who go after you. Your spells can pack a serious punch, your Fireball being the most devastating of all the basic attack spells and your Volcano being among the most damaging of the ultimate spells, so don't be afraid to show that power to anyone who'd think about underestimating you.
Charnel's creatures excel at draining health and mana from enemy creatures. His demons, the Netherfiend and the Hellmouth, can eat souls to get tougher. His spells also drain mana and health from the enemy and weaken their regeneration.
The strategy with Charnel is to feed your demons souls and, when they get strong enough, revive them with the Animate Dead spell each time they die, since they retain their power.