Does Puissant Art count against a maga advancement from reading a summa? Does it count in favor for how well she can write a summa?
The description of the Puissant Art virtue states:
You add 3 to the value of one Art whenever you use it. This means all totals in which the score of the Art is part of the total. It does not apply to Study Totals.
Note that it does not increase the actual score in the Art, it only increases the effective value when using it in a (non-Study) total.
Going now to the rules on summae, we have
A student may continue to study a summa as long as his score in the Art or Ability that it concerns is lower than the level of the text.
When a summa is started, the level of the finished book is determined. The level may be chosen freely by the author, up to half of her score in the appropriate Art or Ability.
Note that, in both cases, your ability to study from summae and the level of summa you can write are determined solely by your raw score in the ability, not by an Art total (or other total). Since Puissant Art explicitly defines "whenever you use the Art" to mean "all totals including the Art", and this is not a total, Puissant Art has no effect on your ability to create or to study summae.