OK, just finished the book. I'll be re-reading it, but here are my first impressions:
The Pred in the book is, physically, very different from the one in the movie, and not just by having only three toes and three fingers (I'm assuming the book was based on the original screenplay as it was written). Nearly all the technology and gadgetry we associate with the Predator is missing--there is no plasma cannon, no armor, no bio mask, no laser-sights, no light-bending cloaking mesh, no wrist blades, no bomb gauntlet. The only weapon is the spear (which, ironically, never appeared in the finished movie, but was later resurrected in P2). Instead of using technology to bend light as camouflage, the original concept here had the Pred's skin itself act as camouflage by its ability to change color and texture like a chameleon or an octopus to blend into its background. The Pred also had the ability to change its body form using cellular control to re-arrange its tissues--there is a scene in the book where the Pred alters its body shape into that of a bird and then flies off in pursuit of Anna as she flees. (The book also mentions that this gave them the ability to construct new bodies if they became sick or injured so they had extraordinarily long lifespans.) The Pred's heat vision is still there, but is entirely biological, there is no bio-mask to let it change frequencies. Instead of wristblades, it has large natural clawlike spurs on its ankles and wrists that it uses as weapons. If the Pred had been depicted in the movie as it was in this screenplay (apparently the CGI technology of 1987 wasn't up to that task), it would have been a very different Pred than the one we know now---more alien, less humanlike, and less technological. This original version of the Pred would have been more interesting, I think, in many ways. But we'd probably not be making Pred costumes right now.
But the book version does do a good job of explaining what the Pred's motivations are. He is, first and foremost, a hunter (and indeed the original screenplay, as written, was titled simply "The Hunter"). In the book's opening scene, as the Pred's spaceship approaches Earth, he is looking through a computer catalogue of Earth species--big cats, bears, elephants, then humans, then specifically at armed soldiers. "Here was a creature modified and trained for a single function", the book explains, "to kill--exactly the creature the predator sought, the challenge worthy of his own vast skill, a kindred spirit at last, a reason to exist." The Pred acted like a British big game hunter in Africa. It picked the most dangerous quarry it could find. It had a trophy room to display its prowess--the book describes a scene where the pred takes a skull and spine back to its ship to place in its trophy room. But the Pred also had a scientific interest too. In many of the bodies, the Pred doesn't bother with trophies and doesn't take the skull/spine---he's interested in understanding their structure and biology. So in some cases he took the bones, in other cases he took just the internal organs--all for study, like a field biologist might. "Only the lower species ever died on the alien's home planet", the book explains. "The higher forms so endlessly transformed themselves that they never inhabited a body long enough to die. They sloughed themselves like snakeskins. Therefore the creature dissected these killer soldiers as dispassionately as a clockmaker might dismantle an unusually subtle timepiece. Obsessively it probed to locate the center of man's identity by analyzing every millimeter of flesh and bone. Though it hadn't succeeded yet, it had already figured that the skull and perhaps the spinal cord were crucial pieces of the puzzle. The rest of the body was patently clumsy and unimportant--discardable, like packaging." So like a human big-game hunter in Africa who sent carcasses back to anatomists in England, the Pred took his trophies but also donated his kills to science. As the book puts it, "It became evident that this was only one of many such trophies displayed around the room. It was some otherworldly equivalent of a big-game hunter's headroom, walls covered with elephant tusks and moose antlers. Yet it bore a kind of purity too, like a scientist's lab or the inner sanctum of a temple."
The book makes it clear that this particular Predator had been to earth before, a thousand years earlier, hunting Aztecs in the very same spot in Central America, and there are references to both the Aztecs and the Sioux treating the aliens as gods from the sky---the basis for the later AvP movie.
So, to sum up: physically, the Predator depicted in the book is very different from that in the movies. Much more alien, much less technological. But in its actions and its motivations, the 1987 movie accurately reflected the intentions of the original screenplay--the Pred is a hunter. It's not interested in war or conflict, it is a big-game hunter seeking the most challenging prey, both in the hunt and in its quest for scientific knowledge. That is, sadly, a picture which all of the later movies began to move away from, until by the time "Predators" was made, the Preds had become mere serial killers, motivated solely by blood lust. It is a huge divergence from the original conception. Me, I find the original conception far more interesting.
