A ghost and a toothpick

So, whatever your build, Magic Spirit is always one of your spells. It falls under I think "arcane" spells which are available to you no matter what kind of spell books you have.

The first thing you should do as soon as possible is cast Magic Spirit to start exploring the map because you can't see anything thanks to the fog of war game mechanic until you go look.

If you choose all death books, it takes a few rounds to come up with enough resources to get a Shadow Demon. You will want a Magic Spirit to go with it anyway to double its travel speed.

So get a Magic Spirit and start looking around. Especially in the early game, some towns and ruins are unprotected and you can take the loot with a pathetic Magic Spirit. 

Some are lightly protected and I get a kick out of figuring out which battles I have some hope of winning with a ghost and a toothpick as my army.

Remember to save scum. 

So if you can grab up resources with a Magic Spirit, it can fast track your ability to get a Shadow Demon and have enough resources to keep it and still have maneuvering room to do other things.

Because my recollection is you can lose the Shadow Demon if you have insufficient resources to pay the maintenance costs.

If you have a pile of Magic, you can deficit spend for short periods while looking for ways to beef up your income in that regard. 

There are trade offs to be made at times. You need enough gold, enough food for normal troops and city populations and enough Magic. Managing balancing those resources is a big part of playing it at all and part of what makes it easy for me to view it as a civilization building game.