It'd be a problem for the sorts of sagas I run, which mostly just highlights that the sorts of sagas you like are different from the ones I like. To me, having the major boon hand the covenant one of the two or three best books ever written on a topic will lead to suboptimal results, as it will tend to mean that every magus in the covenant reads it, because, while it may not be their area of interest, the benefits are just too good to pass up. It also means that, until everyone has exhausted that book, there is a subject that the covenant has absolutely no interest in trading for. It would also, I think, paint a target on the back of the covenant: a bunch of young upstarts have one of the three best books on some art ever written... why wouldn't a Flambeau archmage find a pretext to come turn them to toast?
Also, if the equivalency is "a major boon should give you one of the best books ever", why not apply that same thinking to other areas of covenant creation? Should a single major boon give you a level 9 aura?
Overall, I think it's far better to scale a major boon such that it is quite beneficial, without being exceedingly rare.
But I suspect, if we continued this discussion, we'd have other broad disagreements about the power level we prefer to play at.