Zero Sum Game was a quick, tightly-plotted thriller, which introduced us to Cas Russell, Rio, Arthur, and others. Half Life delivers as the next book in the series, by giving us Cas trying to keep a promise she made to Arthur–to stop murdering people. Several huge threats build through the novel, testing Cas’ promise to her friend (Cas struggles with the concept of having friends now, as well, which is awesome). Cas develops as a character through this book, but this character growth doesn’t compromise her mathematical power or the action, which comes to a tense end. An enormously entertaining book. Can’t wait for the next one.