My strategy these days is to start off etherealized and support my main stack while they look for items and spellbooks in lairs/dungeons. I like starting that way because it costs 5 deity ability points to start materialized, which is a waste of starting points, imo.
Once I have some basic offensive and cc spells researched and enough meditation and battle mana points built up, my B unit stack is about ready to roll out and my A stack is strong and has magic user units that can cast spells on their own. So A stack deity support is not really needed anymore.
At that point I materialize and use my deity to level the B stack and himself up to around level 5. At any point if I need to defend my cities or my A stack, I can just cast etherealize (which is instant) and on the same turn I can defend or whatever. Or, I may also decide to etherealize again if I find some new spellbooks and want to research the juicy spells inside. Then once I do that I materialize again. So, I bounce back and forth between the two states a lot after the first 100 turns or so depending on what I need my deity to do at the moment.
About the starting number of spellbooks: I never bother to research the spellbooks themselves, just the spells in them. It’s so much faster. For that reason I only start with 2 spellbooks. One of each spell school I want to start with. Then I pick the ‘meditator’ and the ‘battle caster’ deity abilities. They give a big boost to the speed of increasing meditation and battle mana. I can always find the spell books in-game but I can only get those sweet boosts at startup. As an example, I started with 1 fire and 1 holy spellbook in my current game. Now, at turn 385, I have:
So, I’ve found all but 2 of those books in dungeons. I spend so much time in dungeons looking for old books that I’m a regular Indiana Jones.
I used my fire books to summon a baby red dragon early on and put him in my A stack. He evolved into a young red dragon which are great because they get the fireball spell at level 5. He’s 5 levels away now from evolving into an adult red dragon which will be pretty fun. From the Holy spellbooks I learned summon angel and have one of those in my main stack too. I takes forever to evolve them to archangels but I just found enough Holy spellbooks to get the summon archangel spell so I’m currently researching that.
Archangels are great because they can summon angels. And angels can summon lesser angels. And all of them can heal.