You know, rather than focusing on whether Authorities are low, high or infinite level, low or high quality, the interesting thing about them is that they are at the origin of their field. As such, there should be a plethora of commentaries on them, which require you to have read the original book first. That is be what makes the difference between a book by Obscurus Ex Miscellanea and one by Bonisagus the Founder. To make any kind of serious attempt at studying Magic Theory, you just have to have read Bonisagus. No other book can do.
That's one of the complaints I have, as far as flavor goes, about the book system: in most cases, books are completely interchangeable and you have little to no reason to seek a specific book so long as you have something with similar characteristics - they exist in a vacuum. With commentaries in play, famous books become attractive because of the body of literature that surrounds them (thus justifying their fame), rather than merely about their intrinsic qualities.
That is what makes Authorities authorities.