The Host, by Stephenie Meyer.
I took a break in the middle of re-reading Meyer's Twilight saga to read her "other" novel, The Host. I liked it a lot better than I though I would. If you just really love teenage vampires, you might not enjoy this one so much. Likewise, if you are a hard-core sci-fi buff, it might just not be science fiction-y enough for you. But if you like Meyer's writing, there are plenty of similarities between The Host and Twilight; a young, vulnerable heroine with an old soul who is selfless to the point of self-sacrifice; a bizarre love triangle; and two groups of people - humans and aliens - who, though natural enemies, find a way to get past their prejudices, understand each other, and work together.