To me, it's quite simple:
If a book is good, it won't be a secret for long.
If it's not a secret and good, it will be bought and sold in large quantities.
If it's good, it won't be rare.
That goes for ALMOST everything in life. If something is so rare that it's extremely expensive to aquire, it may be a nice collectors item, but it probably won't be of much practical use. We're talking consumer goods here, obviously, not luxury items.
Aiming- and other instructional pool books largely seem to fall into two categories: They're either too simplified to be of much use, or they're too complicated to be of much use. Of the two, the simplified ones tend to work a little better, since they are easily remembered and the brain of the user fills in the details. This allows for subconscious decision making, which is generally more often right and less fatiguing. Unfortunately you won't be making many shots with this method from the beginning, since the increments of aiming are usually too coarse and it takes time to fill in the missing increments. So it takes years to become proficient.
There is a third category, which is the well intentioned introspection and then writing down, of an authors ideosyncratic method. This hardly ever works for anyone and generally seems to be at war with the laws of physics, which as we all know is a war that cannot be won. This kind of aiming are more likely to hold you back than help you, IMO.