Prayers of Sigmar

A little confused. How does a warrior priest learn more spells? He starts off with one generated randomly, but it says that Witch Hunters & Sisters of Sigmar can't purchaes tomes of magic, so I'm at a bit of a loss.

When he gets a skill advance he can choose to randomly select a new prayer.

From Campaigns, Advance Rolls, Heroes, "New Skill. Select one of the Skill tables available to the Hero and pick a skill. If he is a wizard he may choose to randomly generate a new spell instead of a skill. See the Magic section."

From Magic, Allocated Spells, "Each wizard starts with one randomly determined spell, but may gain more. Roll a D6 and consult the appropriate chart. If you get the same spell twice, roll again or lower the spell’s difficulty by 1."

A Tome of Magic is a nonstandard way to gain spells.

Much obliged.