Just finished reading the third book (i'd read the first two a while ago before the third was out ad forgotten about it until this thread), it's a great story, and i can't wait for you to write the next series (i am actually quite surprised these books were released to be honest, considering the underlying cause of the zombie virus and current political situations with those involved, i would be too scared to publish in case my name ended up on a list somewhere lol), looking forward to reading more about the road angels (Jessies) exploits after the third battle of lakota.
There's a lot you could do with a game from this series, but for the purpose you need it, i would keep it short and sweet, there's no point having the game actually follow the story, but have it modeled in the stories style with hints at the story line, involving key locations.
Maybe something along the lines of setting it after everything's settled and some travelers have come across three flags and gotten themselves trapped inside and have to blast their way out, looking for ammo and supplies, maybe a few minor puzzles to unlock etc.
Or if you want it set in the timeline of the story maybe set it in the final part of the 3rd battle, you are alone defending a section of the wall and have to kill zombies approaching or die trying.
Something simple and uncomplicated would be best.
I would be quite happy to make a few of the models for you when i get time, but i'm not sure i would have time to make an actual game, in fact i've been sitting at work mulling over the idea for a few days, and trying to picture three flags in my head enough to put it into a 3D modelling program, so if you have any sketches or plans that would be a great help.
Primary Desktop:
i7 7700,k NV1070 8GB, 16GB 3200mhz memory, 1x 2TB Hybrid, Win10.
Secondary Desktop:
i5 4760k, NV960 2GB, 16GB 2333mhz memory, 1x 2TB Hybrid, Win10.
Laptop:
i3, Intel 4000 series graphics, 6GB memory, 1x 500gb HDD, Win8.1.