I love post-apocalyptic scifi, and from that perspective, this book stands out. Surprisingly, it's also a zombie book, but offers a slightly different breed of zombies, created by something the mysterious aliens who dropped off the elevator did. The worldbuilding is well developed, but as others have mentioned, the characters are not. Still,I enjoyed the continuing revelation about the Builders and their imminent return. However, one should note that this book cannot stand alone; you get some answers, but the end leaves much unresolved.