Khohloma,
haha I see the confusion.
The Elemental Specialist will allow you to cast additional spells of the same school. In this case, Serena is Air specialist but you have no Air spells
So you get extra cast and now your book is empty because you have no Air spells. And looks like Magic Arrow is not counted.
Once you learn some more spells the problem will be no more

Also, these Vampires give me a heavy beating you sure you want to attack.