I absolutely love the Zombies, Run! app. To me it's one of the most perfect pieces of media: immersive, motivating (it gets me running, so good job), and the world & story building is interesting. Amazing characters. Plot twists. I love it.
The Way of All Flesh is a tie-in audio book... although it's not so much an audio book as it is a podcast/audio drama. No narrator, just on point voice acting. All the more reason for me to love it tbh. Plotwise it's a pretty decent murder mystery, but what does it for me is the A+ audio production and the fact we get to hear some familiar characters.
You'll definitely enjoy it (a lot) more if you're familiar with Zombies, Run!. I recommend playing (you don't have to run! you can walk, or bike, or sit on your couch!) at least a few missions, or even better a season, before picking up this 'audio book'.