There was an old post concerning "parma magica without books". The idea was that Parma magica is a secret and so magi should not write summae and tractati about it, or if they did, such books must be carefully protected.
IMS, of course parma magica is a secret of the Order, but it's a kind of mystery, such that a magus must be initiated into it during apprenticeship. So for other magical traditions it would be alien. And of course the books (even summae) never reveal the basics of the skill. So a summa on parma magica could not be readily used by magicians from other traditions. Each book could only be used as an insight to integrate parma magica into their tradition, as a "hermetic" breakthrough.
The Order nonetheless considers that such books, if not forbidden, need special security measures. So in most tribunals, the writer (or copist) must ask permission before proceeding, the place of storage of the book must be cleared by an authority of the order (could be the tribunal, the presiding quaesitor...) regarding its security, the owner must agree to maintain adequate, defined security measures and is responsible of its use (for instance, if stolen he must absolutely retrieve it).
In practice, only old and well guarded covenants have been allowed to keep such books - but in most of them, any magus could pay a visit to read them.
In my campaign, and in some online games I've contributed in, I had 'The Red Book'. It was a Summa on Parma (it may have originally been called the 'Balm of the Order' or something like that), decent, not earth shaking or anything, but the original and all its copies were red, but didn't start out that way. The legend was that the original copy of this parma book was stolen by a small hedge tradition and when the Order went out to find it, they slew everyone, and the oldest Hedgie member cursed the Order for causing so much death over a book. That book then turned red with the spilled blood as did the later copies of it. The other legend was similar, except that an Order member was caught trying to sell the book to outsiders and as punishment, the Hoplite who killed him, literally beat the magus to death with the book, by using Rego to slam it into him repeatedly and the red was from the betrayer's spilled blood, a lesson never to betray the Order.
But either way, the legend alone served to warn young magi of the importance of the Parma and to keep its secrets, as well as the great lengths the order will go to avenge itself. Whether or not there were special laws in my campaign, the players didn't know, but the legend caused them to take precautions for the handling of that book all on their own without any further prompting.
An example that is similar is Mystery House knowledge or initiations scripts. I have a summa of Merinita House Lore (L8Q11) that forces the reader to have a base of knowledge in the Lore to just understand/translate. It is fairly useless for someone with a 1 in the ability. If the Order banned Tractati about PM then this could work. Or if the Tractatus had a knowledge qualify just to understand what was in it ( of which I am in favor for all tractati).
Quisque Semita - House Merinita Lore Summae L8Q11 - Written in strange characters and perhaps another language, this book has much of the knowledge of House Merinita. The reader must have House Merinita lore of at least 3 to gain full use of the information. The quality of the book drops by 5 for each point below 3. A character with Merinita House Lore of 2 treats this as a quality 6 book.
First, not only- plus of course obtaining the book. Of course realistically any barrier can be overcome- writing in language coded to require magic theory to read can be overcome by learning magic theory, for example...
Indeed but you must admit that the knowledge of Latin is more prevalent than the knowledge of Magic Theory. If a Book needed MT>4 to understand then the hedge magi must steal another book on Magic theory as well.
or find a way to buy one- they aren't nearly as restricted as a text on PM.
Personally I think rules for an "advanced summae" where you need to have a certain level in the subject to make use of it would be interesting- but obviously there needs to be some tradeoff (aside from security) for making a book that is harder to use...
4th ed had the third book type - after Summae and Tractatus - called Libri Quaesitionum: A book of questions as well as answers, rated for Quality as well as target level. Every point by which your Art or ABility level differed from the target (higher as well as lower) you recieved a penalty to study totals.
While this may actually make sense, it was additional book keeping and more micro-managing in optimizing the reading order of your library's books on a given subject. I think a LQ in general had higher Quality than a Summa, but it was read-once-only. There could also easily be reduncancy if you had both a Summa and a LQ.
IMHO Ars Magica works just fine without them. Although in the many years I played 4th ed (the first edition I actually played) I had no problems with them.
With Parma, it's mentioned that there's a "final key" needed to use it, which is only taught after apprenticeship is completed. So a book on the Parma that didn't contain that key would be useless until you got said key.
As a Parma summa is almost certainly of immense use to young magi, there's legitimate reasons for covenants to have one and there may even be a Root of the Parma somewhere.