To most repliers:
The main thing to keep in mind is that this system is supposed to be simple. You can tell based on the current examples that are listed in the OP. The more complex the spell, the less likely it'll be used.
@ TassadarZeratul
In that case, looking at your summons, it's best that damage based summons are OPTIMIZED for that particular tome unit, but still usable for other ones. Otherwise, some tomes are only used to summon a unit and are just...switched out immediately. It makes the tome unit itself pretty worthless.
1) Terran infantry are fine. Low hp means they die fast as medic can only heal 1 unit at a time. They need to go into a bunker, which needs a scv repairing to stay alive long enough. Infantry then benefit greatly from a SCV being around, but can still be used from all other tome forms. Just control their max number.
2) Protoss Reavers and Dragoons can be easily modified. Make most of their health in shields. Allow maximum of 1 Shield Battery. When the Probe is out, always set shield battery energy to 100%. When the Probe isn't out, let it do whatever (as in, it will run out of energy now). Reavers and Dragoons are now usable, but benefit more when Probe is around. Just control their max number.
3) Infested Terrans and Scourges are fine. They're suicide units. Just control their max number. The same goes with Broodlings as they will die over time. Just control their max number. A unit that suicides or dies over time encourages players to switch back to the original tome unit, but they don't have to.
4) One problem that I can see so far is the Zergling. It gains no benefit from the Queen and you aren't encouraged to switch back to her. My suggestion would be to remove Zerglings and give the Queen an energy refill option that costs gas. This combos with Broodlings as she can cast them far more often. You still have to control the max number of Broodlings though or this could be overpowered.
5) The other problem I can see is the Hydralisk summon. Again, it gains no benefit from the Dark Archon being around. Don't give Dark Archon form a summon. Think of something else. Maybe give him ground hallucinations? Or give him an attacking unit that does 0 damage while managing its maximum count. It'll be difficult to combo with maelstrom though.
Tome of Firestorm Sucks, Remove it NOW:
1) Emp may be inconsistent to trigger and possibly requires a dedicated computer player. I'm surprised you're doing this. It makes Vessel EMP tome redundant. Coding stacked Irradiate is more consistent and promising. Remove this ability. If you really REALLY want a trap similar to do, you can just MANUALLY set shields to 0 and energy to 0 of all units in a location, but it still is redundant.
2) Only three spells? What was that about four?
3) How is the "high damage splash" going to be dealt?
None.