I feel 25 MP is very little. Assuming that the priest has maxed mana at 385, that means that the Priest can spam that space bar a total of FIFTEEN TIMES (900 HP in heals), not to mention the potential of having Rings or armor that can give you mp. If a priest has a GSorc and a Sphinx, that gives the priest 540 MP, allowing the priest to use the tome 21 times, which means that the heal amount is 1260 HP. Now, Imagine a priest with a Divine Pet with level 100 Magic Heal, sitting there, spamming at 8/8 Knight who is standing still. The knight would literally be unkillable. Now, you may be saying “But there’s Projectile Speed!”. If the priest stands close enough, but not too close, when the knight starts to take a bit of damage, the projectiles will start coming in. Even at level 20 Average, the priest will be able to spam that space bar 11 times, resulting in 660 health healed. The priest would just have to walk in the direction they are shooting in and, because the projectile is so slow, they can walk through it healing a few hundreds of health at a time.
My Suggestions:
-Increase the MP cost to 60-70
-Have a bit of a cooldown on the tome (2-4 seconds)
Other than that, I like the Idea!