Annihilation is one of those novels that you can't tear yourself away from, even though you KNOW that you'll have nightmares because of it. That's because VanderMeer has constructed a dense psychological narrative that develops against a backdrop of unknown creatures, contamination, murder, and psychological manipulation. The more you read, the more you know that you don't know anything- none of the characters have names, Area X (which they are sent to explore) is a big mystery in itself, and even the narrator herself admits that she doesn't know if anything she's been told about her expedition is true. So basically, you accompany the narrator ("the biologist") into an unknown and increasingly frightening wilderness filled with moaning creatures and living words that grow on walls and ALL you want to do is hold onto something that you DO know (i.e. the narrator). After all, she's supposed to be your guide, your companion, in this area that has been cut off from human civilization and possesses a seemingly lethal secret.
We learn that previous expeditions into this area have not ended well (suicides, murders, etc)., but it's the expedition that came before the one we're reading about that is the most terrifying: apparently, all of the expedition members returned to their homes from Area X without knowing how they got back and only as shells of their former selves (no really- shells. The "real" people were probably killed by the creatures of Area X and their bodies replicated). They all subsequently died of cancer. Our biologist narrator was married to one of those explorers, and volunteered to explore and catalog Area X for a variety of complicated reasons, including her need to understand what her husband was looking for and her own obsession with isolated natural environments and the life forms that inhabit them.
Early on, the biologist is "contaminated" by spores from an unknown organism and from then on, the reader can never be sure who is actually speaking: the biologist herself, or the thing that she is changing into. Reading Annihilation is like trying to walk across quicksand: each step threatens to send you into an abyss, but you keep going somehow, hoping to find solid ground. At the end, we are left with many more questions than answers, but one answer we do have is the most terrifying of all: the border that encircles Area X is expanding. Is this some kind of alien invasion? If so, is it malicious or simply a case of one organism using whatever it encounters as fodder to survive? I guess we'll just have to wait for the next two novels for some answers. Hopefully, there will be some answers. Otherwise, Jeff VanderMeer will be getting some angry emails from me.