Plague War suffers from the same problem as John Scalzi's The Ghost Brigade: it's a sequel that isn't as immediately gripping as the first book in the series. Plague Year hooked me after the first sentence, but I stalled out after about 30 pages with Plague War. I had put the book aside for a few months while I devoted most of my reading time to comics and picked it up again yesterday because I liked the first book enough to want to give it another shot.
I'm glad I did: while the first 30 pages dragged a bit, the next 250 flew right by. Part of the issue is that the action starts up immediately after the events of Plague Year, so instead of dropping us right into the middle of an obviously desperate situation as in the beginning of Plague Year, Carlson has to spend some time reintroducing us to the characters and the world before we can really grasp all the dynamics of what's going on. It all feels a little repetitive, but eventually it does take off.
It still isn't as solid as the first book. Plague Year gave us characters who, in true Romero fashion, often made the wrong choice for the right reason, while in this book the protagonists have turned into more conventional action/adventure heroes, and they're able to get away with things more often.
In my favorite apocalyptic/post-apocalyptic fiction, the characters are faced with situations where all the options available to them are undesirable in some way. No matter what they choose, they're faced with losing something. (Early episodes of Battlestar Galactica work this way, for instance). Here, though, there are too many situations where the true costs of the heroes' actions don't seem to be taken into account: Carlson is too focused on the benefits.
This is part of a larger, general problem with adventure fiction - especially serial adventure fiction: the tendency of creators to be overly protective of their characters. Not just protective in the sense of "keeping them alive", but also keeping them from doing things the audience might not find appealing. I understand why creators feel they need to do this: serial fiction is generally character-based - rather than situation-based - so protecting the characters - keeping them healthy and likable and attractive - is seen as being necessary to ensure longterm commercial success. However, I don't think it's a coincidence that the last few years have given us critical and commercial successes like Lost and The Walking Dead that gain a lot of their oomph from not treating their characters with kid gloves, while similar shows that tried to keep everything "just the way the fans like it" (like, say, Alias) stumbled.
On the other hand, Carlson has a pragmatic p.o.v. towards romantic attraction which gives the developing relationship between the lead characters a real edge. This is definitely a refreshing element as too often even the hardest-nosed adventure fiction is slips into the most obvious Hollywood sentimentality when it comes to romance.