Affinity tomes contain detailed reports and information gathered by magi over the years pertaining to a specific affinity. With the exception of the General Affinity tome, they will,when read, give a 10% boost to that specific affinity, while simultaneously reducing your progress in all other affinities by 10%" - Arcane Compedium's Description with some minor vocabulary/grammar changes.
More about Affinity Tomes[]
Affinity Tomes in Ars Magica 1.7.10