Spells disappear from the spellbook.

To help us understand the issue, please attach the save files from BEFORE and AFTER the spells disappeared to your message. Specify which hero the spells are disappearing from and which spells are affected. Also, please specify the exact version of "Heroes" you are playing, and if it's a mod, which version of the mod. If the game was downloaded from the internet, please provide a link to the source.
The most likely cause is an issue with your version of "Heroes".
It might also be worth asking: did you turn the pages in the spellbook? There was a recent case where a "disappearing" spell was actually just located where it should be. Or maybe you accidentally switched between the magic school tabs – that could also cause the spells to "disappear"?
I've attached a screenshot for your reference.