Since it is a sequel, it can probably be left unsaid that spoilers for the first book are present.
While the novel kept me up late at night because I wanted to know what would happen next, I didn't enjoy it quite as much as The Passage - after all the build up, the ending seemed a bit rushed. However, I quite liked the build up. In both The Passage and The Twelve, some of the chapters begin with a heading with a date of 1003 AV (after virus). showing that whatever follows in that portion of the novel was presented at a conference. As a result, no matter how dark the novel gets, the reader can assume that some type of civilization has survived. I actually quite enjoy the idea of papers being written hundreds of years after an apocalyptic event and presented at a scholarly conference. This novel begins with a prologue, presented at this conference, which is basically an account of the previous novel written much like stories in the Bible. It really is a great way to jog a reader's memories of everything that occured in the previous novel (of course, I read the novels back to back so I'm not sure how effective it was for someone with a year between the two).
After this quick refresher, Cronin jumps right into the action. It is five years after the end of The Passage and despite various hunts to find the other eleven (one of the twelve having died at the end of the previous novel), they haven't had any luck locating any others, and continue to face the same challenges. Amy is living in a convent with "baby" Caleb who along with Hollis survived the Roswell Massacre. Peter and Alicia continue their hunt as officers in the expeditionary forces, and Michael is working on an oil rigger. Having set the scene and reacquainted readers with all the characters, the novel then flashes back to the year of the outbreak. In The Passage, Wolgast and Amy hid in the mountains as civilization fell. This novel portrays exactly what was happening while they were there, using characters previously introduced, such as Grey, or mentioned, such as Wolgast's ex-wife Lila, and "the Last Stand in Denver." He also gives more background on some characters from the previous novel. I actually really liked these parts of the novel, especially when I saw some of the connections forming.
Once the narrative returns to 97 AV, all the characters find themselves enroute to the same goal, independently of each other. After meeting Anthony Carter, the final death row inmate to become part of the Twelve, in the previous novel, it was nice to return to him, and see how he had evolved in his state as a viral. Unlike the other violent, brutal criminals, Carter was always an anamoly. Given that Cronin introduced the small town of Haven in a previous novel, the revelations he makes here make complete sense within the narrative context of the story. Besides the almost rushed and slightly unsatisfying ending, I think my biggest gripe with the novel was a certain female character's protrayal. While I think it makes sense that a person's brain might shield them from the trauma they were surrounded by, I just expected this particular character to be stronger and more stable. I think the main reason this bugged me though is because it just seemed like such a stereotypical character from certain types of fiction - the crazy mother that blocks out the world. I almost felt like I was reading about the crazy Southern belle in a novel set around the Civil War era. After having so many strong female characters in the novel, it was just disappointing to see him fall back on an old literary trope. Still, while the novel wasn't perfect, it was still a good second novel, with some forward progress, setting up for the last part of the trilogy. More importantly, I still care about the characters, and want to see how it all ends (something I wasn't saying after the second novel of The Strain Trilogy)