A Warrior Priest or Sister of Sigmar with a holy tome can add +1 to the score when determining whether he (or she) can recite a spell successfully or not.
Lore
Books of prayers and descriptions of the holy deeds of religious heroes like Sigmar Heldenhammer are copied by hand in the scriptoriums of Sigmar and Ulric, and given or sold to the faithful. Of these tomes, the Deus Sigmar is the most common and well known, but other texts such as the Scriptures of Sigmar are also sold to those who follow the faith. A holy man can recite his prayers from such a book, strengthening his faith and belief.