The Partials Sequence Complete Collection

Dan Wells builds a scientifically rigorous post-apocalyptic world and centers it on a genuinely hard question about what makes someone human — that's more than most YA dystopia manages. The mandatory-pregnancy storyline is handled thoughtfully but is prominent enough that parents of younger teens should know what they're getting into; this is solidly a 14-and-up read. Pacing sags in the middle volumes, but the series earns its reputation as one of the more intellectually serious entries in the genre.