My attempt to spiff it up a little.  I do like much about it but it is a little clumsy.

To access:

• When you are on a caster or your Sovereign just use the existing spell tab. 
• When you are on a city or clicking about the map have a similar spell tab in close to the same location if not the same spot. 

When clicked, the Spell Tab, immediatly brings up the spellbook.

Minor spellbook changes.

• Have two TABS on the top      1:Known     2:Learning
• On the Right side of the book have Tabs for the classification of the spells:   Favorites, Summoning, Life, Death, Fire, etc..  (Maybe even an All spells tab)
• On the Left Side of the book have it display whatever tab you have chosen. and the corresponding spells on that right hand page.
• When you open the book it opens to (1: Known) you can either double click or rightclick (whatever you guys think works) to cast a spell. I would have it open always to the (Known) tab, but have the pop up notice when your learning spells take you to the Learning tab. or maybe have the spell learning bar on the bottom left take you to the learning tab if you click it.
• When you single click a spell the description and stats shows just like it does now.
• Pages turn like they do now, perhaps the icons are a spot smaller so you can fit 4x5(20) or 5x5(25) on a page. 
• The description side of the book is nice.
• If possible have the transparency of the book background adjustable, would be nice but not that critical.
• The little overlays showing that something is not available to cast, do to mana or a timer or the effect already exists should be visible when the spellbook opens.

Whether you are on your units or not you would be able to access and cast a spell rather easily.  If you could fit 20-30 icons on a 'favorites' page you should be able to do most casting in 2 clicks. 

I like the system you guys have set up, and these are just a few thoughts to try and help streamline it a little.