HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

Found a rather interesting bug (it's hard to call it a feature). Having a Wizard, but not having a spellbook, I couldn't cast Oil. Although Travel and other utility spells are cast normally. And the very definition of a Wizard sounds like...

Well, it has always been this way. The same applies to scrolls. In the early versions of the game, it was possible to cast combat spells without a spellbook, but this was fixed. In Heroes of Might and Magic V, they changed it so that utility spells also cannot be cast without a spellbook.