Just want to clarify spell sucesses. Is the +lore assets towards spell effects a guarranted success or an additional dice to roll after your first test that goes towards successes?

It is a 1 additional die you may roll when resolving a spell

It is a 1 additional die you may roll when resolving a spell

I'd rather say +X die, because the Arcane Tome adds +2 and Zanthu Tablets (IIRC) adds +3 dice. But that's just nitpicking and wordplay.