When I was a kid, I was a huge fan of novelizations of films. I honestly can't tell you why, but I bought a lot of them, often at the used bookstore my dad was a fan of and to which he took me a number of times. Sometimes, it was films I'd seen and loved, like The Rocketeer. Sometimes, it was films I'd seen and been disappointed in, like Dick Tracy. Others, like virtually all of the Star Trek film novelizations, were books I just picked up because I was familiar with the property. I don't own any of them, anymore, though, so I guess ultimately they weren't that important to me, but it was just a weird little phase I went through.
Anyway, as for the Alien novelization: it's a light read, as you'd expect. It took me perhaps three hours total to read and the plot doesn't deviate in any meaningful way from the film's, aside from including a couple scenes cut from the film version (the goodbye scene between Ripley and Dallas, for example, which made it's way back in on one of the 21st century director's cuts), and the derelict ship's pilot being absent. I found it interesting, though, that Foster added a single eye (or eye-like structure, as he called it) to the face-hugger's description. While reading the novelization, I at first imagined this merely to be a mistake on the part of the narrator, but when I thought about how they're depicted in the films, there's simply nothing that could be construed as being an eye of any sort, so I suppose that's simply Foster's take on the creatures. Additionally, the fully-grown alien has eyes, confirming this being how Foster saw them in his mind, which in the scene where Brett dies, he momentarily mistakes for Jones's eyes glowing in the darkness. Foster also mentions suction cups on the creature's undersides, used to attach itself to Kane. This I honestly found kind of silly compared to the way the things attach themselves in the films.
These are all things, however, that I imagine Foster used either from his own imaginings of written descriptions given to him, or perhaps from early concept artwork for the film - which of course went through many iterations before settling on the versions we saw on film. I'm glad that the final film versions did not have eyes, however, as frankly, it's far more chilling to see that shiny, blank skull "peering" at the camera and/or characters, than "giant, glowing yellow" eyes would be.
What surprised me was the tone of the novel - maybe it's the period in which it was written, or maybe Foster's own sensibilities, but the horror is decidedly absent, compared to the film. The tone, instead, is the sort of dry, by-the-books sci-fi you'd expect of 1950s/60s commercial science fiction prose. A lot of acronyms we're never given the meanings of, a lot of description of step-by-step actions taken by the crew as they perform specific tasks on the ship. Maybe it's meant to add realism, but while it works in a film, it really bogs down the narrative in prose, as it takes you out of the story and makes the reader feel like an uninformed observer who's suddenly become aware they don't belong on the Nostromo's bridge. The scene-changes, where the film frequently flicks back and forth between various crew-members, are badly handled, too, taking the reader out of the story - rather than line breaks (of which the novel has only one), the next paragraph will simply be focused on a different person in a different place. It was jarring.
In addition, the novel feels unbalanced in terms of the "acts" it's divided into. We're 110 pages into the book before Kane is back aboard the ship and placed in the autodoc and another sixty pages before the chest-burster (which has limbs, in this, as opposed to the worm-like appearance in the film) makes its appearance, by which point the book had begun to feel like it was dragging. The remainder of the plot plays out in less than a hundred additional pages, but contrary to feeling rushed, it actually feels lethargic - as if very little is happening and it's being padded out. That is, until we get to the final climactic scenes where the last of the crew--save Ripley--is killed and Ripley makes her escape - all of which, including Ripley's finally besting the xenomorph, is crammed into the last ten pages of the book. It's just badly-balanced plotting.
All of this sounds like a lot of complaining--and it is--but don't get the impression that I hated the novel, because I didn't. It wasn't as good as the film, but it was interesting to take a look at the story from a different perspective and in a different fashion. If I'd never seen the film before reading the novel, however, this book probably wouldn't have enticed me to watch it - so I'm glad it happened the other way around.