By midgame, my spellbook tends to have a healthy amount of spells available, which makes it harder to comfortably use over time.
My suggestion is to have a separate page for spells that we tend to cast only once per game (or not much more than once).
Also, it would be a helpful spellbook feature to have a list of your races pop up on mouseover on Race Transformations that have/don't have that specific transformation - this would be a very handy UI improvement when you are the keeper of multiple races.