“Flight to Salvation” – “Deeds and Paths of Darkness” [4th campaign, 1st mission]

It seems that, as mentioned above, you need the Phoenix, but I didn't get it in the first mission, so it makes sense to take the Tome of Summoning as a bonus at the beginning of the second mission, presumably for those who also don't have the Phoenix summoning spell.

In the first mission, all four cities have a primary summoning spell, plus there's a treasure room from which you can receive magic spells of level 4-5 each month. You can learn all the summoning spells along with other schools from the Genie, by taking the "Skill Academy" bonus in the second mission. However, the tome doesn't carry over to the next missions, so it's not very useful.