Many times a party will come across a puzzle in some old temple that requires the understanding of some ancient language or some Aptitude that is not as common as say acrobatics. So they wonder, how rare is this information?
Certainly somebody had to have this knowledge to build said puzzle. So we must look at rarity with perspective as to the people that hold knowledge. The everyday Praelian, learns their job and the appropriate Aptitudes therein, maybe an occasional hobby or other Aptitude in their spare time, but most of their lives are spent working for their living.
Adventurers are a rarer breed. They pick up things along the way that others may not. So when they find a book, the Arbitrator must determine how rare this book is. Generally one should think that if an Aptitude is not in the job set of most jobs, then it should be rare. If it is a specialty version of an Aptitude, such as Celestial Navigation vs Navigation, it should be even rarer.
For each rare level, consider increasing the comprehension by a factor of 6. The closer the player is to understanding the knowledge, that factor can be reduced.