World War Z, by Max Brooks
With this book and its prequel "The Zombie Survival Guide", Brooks pretty much brought the zombie genre of horror back to life, if you'll pardon the expression. And it's jolly good.
The author uses the conceit of documenting personal tales for a history of "the zombie war", as a counterpoint to the dry official history of facts and figures, and it consists of multiple "interviews" between the author and individuals from all walks of life who recount their experiences from immediately before, from during, and from the ensuing cleanup after the war. Most of the interviewees have very similar voices, but their experiences differ widely, and most of them have a strong human element to them. And while some are merely about killing zombies, many are about terror, fear, and hope.
By being broken down into short interviews instead of one long narrative - although the interviews are arranged roughly chronologically and there is some interaction between them - it is particularly well suited to reading a bit at a time while commuting with all the other gray city zombies.