How did the writers of Morgan's book predict the alien invasion so correctly?

Question

In the movie no one knew how the aliens invaded. But the book Morgan buys predicts how the alien invasion will play out. Minus the part about small size of the aliens, the aliens didn't fight an airborne battle and it seemed that they came to harvest the planet. The news report, the morning they leave, on the radio confirms it. A lot of what the book says ends up coming true.

Answer 1

You may be giving the book a little too much credit, in terms of the extent to which it's predictions were accurate.

As you noted, it was wrong about the size of the aliens:

MORGAN: "They tell you everything in this book. It says they're probably very small -- like my height -- because, as their brains developed, there was no use for physical development. It says they're probably vegetarians, because they would have realised the benefits of such a diet."

It also didn't strictly predict that the aliens would necessarily wish to harvest the Earth. It merely proposed that as one possibility, along with the alternative proposal that the aliens might visit the Earth as peaceful explorers:

MORGAN: "He says there are two reasons why extraterrestrials would visit us. To make contact in the spirit of exploration and furthering the knowledge of the universe. Or the other reason... they're hostile. They've used up the resources on their planet and are looking to harvest our planet next."

Apparently, Morgan didn't read anything about the aliens' aversion to water in the book either, and sounded sceptical when Graham brought that possibility up:

GRAHAM: "I heard a theory that, uh, they don't like places near water. Maybe, we'd be safe from them near a lake or something."

MORGAN: "Sounds made up."

The main thing the book was shown to get right was it's prediction of what sort of tactics the aliens would employ, if their intentions were hostile:

GRAHAM: "Tell me something Morgan. In that book of yours, did they happen to detail what would happen if they were hostile?"

MORGAN: "Yes. They would invade us using only ground tactics. Hand-to-hand combat. They wouldn't use their technology or fight an airborne battle, because they would know we would eventually use nuclear weapons and the planet would be useless to them."

This could've been based on pure, logical reasoning, perhaps extrapolating from what humans would likely do in the aliens' shoes.

I suspect there was a little more to it than that, though. It seems likely that the authors of the book conducted extensive research on prior instances of alleged alien sightings on Earth. We see textual evidence of such research on one of the pages shown, and artists' renderings of aliens, which bear quite a close resemblance to the actual aliens in the film.

Given this resemblance, it's possible that the aliens may've been scouting the Earth for years -- perhaps decades -- prior to the events of the film itself, and been observed by human witnesses. And if they were, then the theories presented in the book may'be been based at least partially on documented evidence of the aliens' behaviour.

Answer 2

Morgan seemed to derive most of his insight on the aliens from a book -- titled WE ARE NOT ALONE: Evidence of the Extraterrestrial -- he purchased from Carl Nathan's bookstore during the first half of the film.

He'd clearly read it thoroughly, judging by all those bookmarks, and directly relayed info about aliens from it in at least two scenes:

MORGAN: "They tell you everything in this book. It says they're probably very small -- like my height -- because, as their brains developed, there was no use for physical development. It says they're probably vegetarians, because they would have realised the benefits of such a diet."

MORGAN: "He says there are two reasons why extraterrestrials would visit us. To make contact in the spirit of exploration and furthering the knowledge of the universe. Or the other reason... they're hostile. They've used up the resources on their planet and are looking to harvest our planet next."

GRAHAM: "Tell me something Morgan. In that book of yours, did they happen to detail what would happen if they were hostile?"

MORGAN: "Yes. They would invade us using only ground tactics. Hand-to-hand combat. They wouldn't use their technology or fight an airborne battle, because they would know we would eventually use nuclear weapons and the planet would be useless to them."

Carl's wife said the book came by mistake in a shipment, suggesting it wasn't the sort of book they'd normally have in stock. But given the nature of the film's plot, perhaps the arrival of that book in that store was providence, rather than coincidence.