I own the book and have used it exactly as you describe, to get a MoN prequel, along with the scenario in the MoN companion. I cannot pretend to have your depth of knowledge about the Aztec and Mayan cultures, however the book has an extensive section on the ancient cultures of Mexico, and it seems to be a well-researched and good source book in that regard, written with sympathy and not stereotype. The scenarios are also quite good, they can stand alone, or make a little loose campaign, or be dropped into some longer campaign.
I agree that $60 U.S. for a book is expensive, so I cannot comment on value for money, but at a lower price, I would recommend the book enthusiastically.
But let's look a little more at MoN and such. The MoN companion correctly points out that one of the weaknesses of MoN is that it opens with the announcement that "the investigators are all good friends of Jackson Elias." No elaboration of how this came to pass. The companion, at least the proof reading version that I had, tried to solve it, but still opened with the characters knowing Jackson Elias, at least well enough to warrant his sending them a telegram.
So, to solve this problem, I resorted to what might be called the "long lead-in" to MoN. Any scenario can be the right opening scenario, provided the instigating character who involves the investigators:
a ) can be replaced with Jackson Elias and;
b ) survives the adventure. Jackson Elias has to have script immunity for the first (several) scenarios, because he has to be available for the opening of MoN, or there is no point to the effort in the first place.
So, I opened up with a scenario from one of the monographs, where the investigators had to retrieve a manuscript for the instigating character, and I made some whole-sale replacements of characters, assuming that it is a small world among the elite. Jackson Elias has more than one friend. Bradley Grey, lawyer to the Carlyle interests, must have more than one client in his New York law firm, so one of them in New Orleans tasks his good friend, Jackson Elias, in New York, to run down to his lawyer, Grey, to see about getting a manuscript back that had been improvidently lent out. Grey doesn't usually handle this sort of thing, and calls his usual private investigator, who also doesn't handle this sort of thing, but recommends the investigators. Elias, and the investigators, meet at Grey's office, and the investigators are briefed.
The investigators go and get the manuscript, and realizing that Call of Cthulhu manuscripts are usually bad things, somehow manage to drop the last page of it in the local river, though they then fish it out. This ends the first scenario.
Elias and the investigators go to New Orleans, to square things with the patron about his damp manuscript. They go to a speak easy to for dinner to calm ruffled feathers, and gun play breaks out. This turns out to be "Dead Man's Stomp." Hijinks ensue along with a very large fire in a store of medicinal alcohol. This ends the second scenario.
Elias goes off to Mexico, following a lead, but leaves the investigators a copy of his latest book. The patron realizes that the investigators have certain talents and recommends them to another friend who has a problem. This turns out to be the scenario in "Secrets of New Orleans." More hijinks ensue. This ends the third scenario.
Elias calls for help, this being the scenario in the draft edition of the MoN Companion, however the big lead he is working on is not really in that site, but in one of the scenarios of "Mysteries of Mesoamerica." The investigators work their way through both scenarios. This ends the fourth and fifth scenarios.
Elias now goes to get into the trouble that will lead to the opening chapter of MoN. The investigators go back to New York, where they have scenarios six through 'N,' to mark some time off the calendar--Grey finds work for them, his clients find work for them, and we are pulling scenarios out of any handy book.
No rush to get to MoN--we may never even get to MoN by this method, but we build to it inexorably. Trick is to start early enough on the calendar so as to leave enough time.