These aren't necessarily hard and fast rules.
regarding art levels;
Certainly characters with over a 40 in an art will be exceedingly rare. The core book posits 40 as by and large the highest level a character could expect to gat in an art.
This fits pretty well with the game rules as published. A fairly dedicated student of one art migt spend two seasons studying the art every three years of his life post apprenticeship. Looking at an average quality of a tractatus as 10 (from covenants) and 100 years of life past apprenticeship the characer nets 666 xp that is only enough to get him to level 36.
However, if the character has an affinity this experience total goes to 1000 also with if the character devotes one exposure experience points to the art per year he'll end up with an additional 200 experience points (due to the effects of the affinity). 1200 experience points puts the character at level 48.
I don't believe that 2 seasons every three years is conservative, but the other assumptions (no book learner, average quality 10, 100 years past apprenticeship, 1 exposure experience per year) are quite conservative so I can imagine a strong case being made for levels higher than 48 being achieved.
regarding book quality
Elder magi do have much to fear from possible twilight episodes but if they want to take the risk a communication of +5 is accheivable with some effort by the use of creo mentem spells.
When we speak about a book that has a level of 20 adding all three points of resonance to the book becomes feasable. Typically the last point of resonance that is gained by infusing the book with a pawn vis over the course of a season would be painfully imprafctical but if a character is going to study from the book for eight seasons to understand it all it becomes very adavantageous to add the last point of resonance if more than one person will stufy the book.
So here's what I think the best Sumae in the order look like
They're written by a character with an art score of 44 and a communication of +5 (achieved with creo mentem brain steriods).
They have bonuses for scribe, illuminator and book binder. They have three points of resonance bonuses. (would it be possible to gloss the volume for another point of quality ?)
We are looking at level 22 quality 14 (or 15 if glossing works). Or level 18 quality 18 or 19.
But this isn't the theretical maximum. All my example book requires is that a focused specialist lives for 100 years past apprenticship.
A significantly better book would be created using the (significantly less waranted) assumption that a focused specialist with book learner and good teacher lives to 150 years past apprenticeship. (I calculate it out at level 28 quality 17 or 18 ).