All books in the Saga of Stranggore are tractatus. We don't use summae at all.

The main reason for this is playability. A tractatus is valuable to any magus, so it's always good to have in the library. Summae are of value only to relatively low-skilled magi. We don't want players to feel gyped because we spent covenant build points to buy a summa their characters can't use.

Also, we don't really agree with the model of learning presented by Ars Magica Fifth Edition. Having books that cover all relevant material up to a certain Art or Ability level arguably makes some sense for modern textbooks and the like. We feel that medieval books were few enough and short enough (remember, they were hand-written) that every scholar would potentially learn something from every book, even if he had already read similar books (for example, bestiaries). Therefore we think the rules for tractatus have more of the medieval feel we're looking for.