By 'something else' I meant Lab Work, the usual way magi get Exposure XP. Since you perform the Parma ritual every day (twice in most cases, more if you cover others with your parma often), it's very easy to justify gaining Exposure XP in it in a season you would otherwise gain no advancement XP.
Most magi pile their Exposure XP into Magic Theory, which in my opinion is often a bad choice. High Source Quality Magic Theory books are fairly common and easy to acquire.
I like the idea of Parma books, but there needs to be an understanding of the grave responsibility involved in having suce a book. The one I've sometimes used in my sagas is 'The Red Book'.
It's a Parma Summa (I believe L 5 Q 15) so no great shakes, but certainly better than exposure and adventure xp. It was originally called 'The Balm of the Order' in reference to Trianoma's sales pitch. The original book still exists and is owned by a major covenant (I usually make it Coeris or Durenmar). The book itself and all copies made from it are a deep red, the color of dried blood. The legend is that a Hedge Wizard stole a copy and taught the Parma to his small line of wizards and incurred the wrath of the Order who Marched them all. The head of the line was the last to die and he cursed at the attacking wizards that they would cause so much death over one book. The book turned red after that and the original is actually a vis source now. All copies of the book turn red over time, as do copies of the copies.
Another legend regarding the book is that the book wasn't stolen but sold to them by a wizard of the Order. The line of wizards was still Marched and when the traitor was discovered he was beaten to death with the recovered book and judicious applications of Rego Herbam, and it is his blood that stains the book and all copies since.
But the point is, I think the book should exist, but understand it's a grave responsibility.
Technically that wouldn't be a March. That refers to to a Wizard being cast from the Order and hunted.
Since this was a hedge tradition they would simply have been declared war (not War) upon.
I would think that'd be a distinction largely ignored though. I'm not disagreeing that a March is typically reserved for (former) members of the Order, but at the same time Wizard's War is only for members also. And if the legend spreads through out the Order regarding the Red Book, I would imagine they would say the Hedgies were Marched and no one would be confused.
To be honest, I believed that a Parma Magica score of 1 at gauntlet was mandatory (since it's the last thing taught, and they're not taught the final secret until after they've sworn the Oath). Although the inset on page 32 does say "should be no higher" and not "may not be higher," so it looks like I was just misunderstanding again.
But, to the subject at hand. You can't learn Parma Magica unless you have the Gift, so I don't see any reason why you can't have books on the matter. Or even have your mundane scribes learn Magic Theory and then copy the books for you, since it's nonsense to them without that innate ability.
Because not everyone with The Gift is a member of The Order, nor even friendly to The Order, and some are its sworn enemies. It is, essentially, a "military secret", and some Sagas treat it as such by limiting whether it's ever found in written form (which makes some sense if you stop to consider the "security" (or utter lack of it) in some Covenants).
A thought- Parma magica being an ability, most books with a decent level are likely to not have a great quality. Consider a book written by someone with ability 8, level 4, say 11 quality... and given the limited pool of people to write such a book this is likely to be exceedingly rare, which means book quality likely maxes out at 11 to 14, with levels around 3 or 4. Teaching on the other hand one on one gives teaching score+com+bonuses+9, with a com of 2 and teaching 4 means 15 source quality. If you allow the rules from Artes and Acadame you can also divert some of those points into teaching, which means anyone looking to take on an apprentice should consider finding someone to mentor them in this skill...
One advantage magi have, book-writing wise, is their greater lifespan. For a normal person in Mythic Europe a 10 in an Ability is the pinnacle of skill, something only a few could ever hope to achieve. A magus who applies himself, however, can gain multiple abilities at 10 or better, thanks to having 4 free seasons a year and 2-3 times the lifespan, most of which is in good health/vigor.
Of course, there's no guarantee that the guy who gets his Parma Magica to 14 is going to have Good Teacher and high Communication. But if somebody does get their Parma Magica to 14, they'll write a pretty darn good L4-5 Summae.