This is book 4 of the "Black Tide Rising" series.

This note contains spoilers for the book.

Final book of this particular zombie apocalypse.

The intro says it's an unplanned book, it insisted on being written when he thought he was done. Given that Captain Smith was thinking of long-term plans not yet revealed at the end of the last book, he couldn't possibly have really intended to stop there, though. At least, he should have known better.

Jumps around more, gets into politics. The scope is expanding as the number of people rescued and put to work expands. They start clearing Washington DC, and run into difficulties with the underground tunnels providing water and access to too many zombies. They also run into problems rescuing people higher in the chain of command (civilian as well as military; they've got the military side covered after an unfortunate early incident) than the current incumbents. That was predicatble, but it's pulled in the book as a surprise, so that bugs me.

Also the stupid-ass person they rescue is portrayed as a near-illiterate caricature liberal, of the sort that existst only in the minds of people like Ringo. Which he'd managed to keep out of his writing for most of the time, so it's a nasty shock when it's suddenly center-stage. Breaks all the suspension of disbelief and all that.