Spells seem like an interesting addition, given that they can be used in the off-hand and aimed effectively while flying, allowing for aerial combat - finally!
About that, though; I'd like to suggest that you eventually enable a method for players to effectively aim regular ranged/melee weapons while in the air. This could be a powerful ability, so you might not want it unlocked by default. In that case, perhaps you can have a spell-book that can be kept in the off-hand that provides no direct combat ability, but when activated it will lock the player's aim/weapon facing in the air for their main hand, not unlike using a power-swing while grounded. To make it truly useful, this ability to aim in any direction should work both while hovering and while flying around at Mach 2. Casting this spell could drain energy so that this special mode of flying isn't free, but not so severe that fighting in the air is rendered practically impossible. I think you've gotten the hang of balancing things, so I trust that you'd make a good decision in the energy-drain department.
As for the name, given how wacky it'll probably look to have the player be able to face in any direction while flying, you could name the spellbook something that acknowledges that - perhaps "Compendium of all known laws of Aviation (Wawsp Edition)," as an oblique reference to the "according to all known laws of aviation, there is no way a bee should be able to fly...the bee, of course, flies anyway" copypasta made popular by the infamous "Bee Movie."