Just finished a long mage run and man, did I underutilize the fire book. I mainly used water book, which is very good for mele enemies, especially slow ones, and insect abominations, but bad versus archers and jumping enemies(deepones) and enemies that can drag you to them(some abominations and deepones). The fire book bombardment stuns most of the time and lets you charge a fireball in peace, compared to the slow from the tornado, which allows archers to shoot if they bounce backwards, certain enemies to drag you to them in between stuns(sometimes right into your own fireball...) and jumping enemies to sometimes jump next to you. Its also better against boars, since the slow doesnt stop the charge animation, but the stun does.
Also won against Meryme with the fire book(without mines), as it deals injuries frequently. I am probably not going to play mage again so it sucks I only now realize how useful it is.