So, you want to write a zombie novel.
There's two ways of going about this.
The first method is write a novel and put zombies in them. Bam. You've succeeded. Far Cry 3: Blood Dragon has two levels where you meet the ravenous undead. The video game isn't about zombies but they are a part of the story. This, obviously, isn't touching the real meat of the question (pun intended). How do you write a novel focused on the ravenous undead? Not as guest-stars but the main attraction.
Well, the first thing you have to do is define what what a zombie is in your setting and what rules apply to it. While George Romero created the "modern" zombie, the roots of the creature go back much further. The first reference to the ravenous undead is in the oldest writing known to mankind with the Epic of Gilgamesh. Ishtar uses them as a threat to gain herself access to the Bull of Heaven.
"If you do not give me the Bull of Heaven, I will knock down the Gates of the Netherworld, I will smash the door posts, and leave the doors flat down, and will let the dead go up to eat the living!”
Sadly, we never get to see Gilgamesh versus the hordes of the dead and he fights the infinitely less-interesting bull. Amusingly, vampires were more like zombies in their original myths. They were ugly and disgusting creatures who rose from the grave to kill the living. Some small few looked like their old selves and sired children on their (still-living) wives but for a long time, zombies have been a favorite monster of storytellers. There's also the Voodoo zombie, which is a corpse raised by magic as a servant or a brainwashed human being depending on how you want to deal with the idea. This idea isn't quite as popular as it used to be but still has a place in zombie stories as the Anita Blake novels show.
For the most part, I determine these rules to be the qualify of what is required to be a zombie versus some other form of monstrous animal.
* It is dead.
* It is decaying or corpse-like to the point it doesn't have vital signs.
You can do intelligent zombies, mindless zombies, hungry zombies, zombies sustained by magic instead of flesh, zombies coming back because of vengeance, zombies coming back because of a curse, zombies which have destroyed the world, or zombies which threaten only a small fraction of people like Jason at Camp Crystal Lake. There's an infinite number of tales to be told with the ravenous dead and you don't have to stick with the classics. Hell, even Tolkien had the Barrow Wights and the hordes of the dead used by Aragorn to protect Gondor.
Really, the big question is what your zombies represent. This will determine what sort of qualities you want to attribute to their existence. In Night of the Living Dead, George Romero's horde of the mindless dead exist as an excuse to force class-conscious and racially uncomfortable people into a house together. They're a natural disaster that can be adjusted or altered any way the storyteller wants.
In the original Dawn of the Dead, the zombies are representative of the mindless consumerism (pun almost certainly intended) which was afflicting American life. In the Resident Evil films, the zombies represent (however obliquely) the military-industrial complex's continued production of ever-more-destructive weapons for seemingly no reason.
My favorite use of zombies is as a metaphor for plague and sickness. Zombie-ism (for lack of a better word) spreads like a disease from carrier-to-carrier. It knows no difference on class, wealth, race, or age. It brings sickness followed by death and, because of that sickness, the person becomes a threat to the people around them. The fact Zombie-ism is usually incurable only makes the situation more tragic.
What's next to consider is that zombies, by and large, aren't actually characters. They're a plot device. George Romero subverted this by having a couple of his movies show the undead gradually recovering it's humanity. Warm Bodies was all about a zombie as a romantic figure (the humor coming from how ridiculous this is). Before it was the 1993 movie, My Boyfriend's Back, where an intelligent zombie must adjust to eating the flesh of the living to go on a date with the girl he likes.
Perhaps the best use of intelligent zombies is in the Fallout games. Radiation causes all sorts of funky effects in the setting and one is that people killed by it during the war, as often as not, rose as deformed slowly-rotting undead called Ghouls. The thing is, aside from their appearance, they were perfectly normal people and subject to prejudice. Fallout 3 added the fact ghouls were not immortal and eventually degenerated into the mindless undead they were feared to be (but only over the course of centuries).
However, for the most part, zombies are unintelligent parodies of human beings.
So where does this leave the writer? Well, the important thing to realize in these circumstances, the "star" of the work is probably not the undead but the humans reacting to them. This is a delicate balance to maintain because audiences who read zombie books are, as often as not, there for the suspense.
The heroes exist to be lunch until one or more of the group survives (or they all die). So, when crafting the protagonists of your zombie novel, you should question whether or not characters are meant to be expendable or not. By and large, there's usually a "Final Girl" or a small group of survivors to illustrate the dangers of the zombie threat.
This too can be subverted but should only be when you have a reason for it. Peter Cline's Ex-Heroes novels are about superheroes versus zombies. The traditional narrative of plucky survivors against the undead is subverted by the fact the undead don't actually pose any real threat to the majority of them. Saint George is immune to being bitten, Zzzap is made of electricity, and Stealth is simply too talented to be defeated by the undead.
In this case, the tension is from the fact they are attempting to protect regular humans from the undead. They may face non-zombie threats which menace them but there's a class tension which emerges from the fact superior beings are thriving with "normal" human beings forced to exist in their shadow.
How much danger are our heroes in? Who are we willing to sacrifice to the Grim Reaper? A protective narrative places the heroes as individuals trying to save others while a survival horror narrative is about living yourself. The two can be combined, as we see on The Walking Dead, but a real question is how menacing you want your creatures to be.
Bluntly, it is my recommendation you should always kill some of your darlings in zombie fiction lest you undermine the zombie's threat. Zombies are dangerous and nothing reveals this quality more than body count. When zombies have destroyed the world, as in post-apocalypse scenarios, the need to "prove" their danger lessens dramatically.
Zombies don't have to all-powerful and dangerous to serve their purpose. Indeed, one of the appeals of the monster is they're rather crap by themselves. A common element in zombie fiction is the zombies, after an initial period of intense danger, become relatively easy to survive if you know what you're doing.
The traditional Romero zombie is slow because it requires the flaws of humans (overconfidence, treachery, or greed) to make them dangerous. This is why I prefer my zombies to walk rather than run. The thing is, though, zombies don't get tired and they can take all the time in the world (bwahahahah--ahem).
Another element to consider when writing a zombie is whether or not it's a good idea to include psychos. "Psychos", as coined by the Dead Rising series, are individuals who have been driven mad by the events around them or are simply opportunists seeking to profit by it. Man is the real monster, blah-blah-blah. The thing is, that's actually a pretty effective lesson when it's obvious (more often than not) our heroes would be able to survive if they worked together.
Psychos offer an excellent opportunity to provide writers with villains who can talk. People often react poorly in traumatic situations and is there any more than cannibal monsters rising from the dead? People might do something insane like try human sacrifices (The Mist), kill their fellow humans so the zombies are distracted by the fresh meat, or even turn to looting as the breakdown of order gives them a chance to fill their material desires (Dawn of the Dead).
There's also the question of the "Z-word." Zombies are pop-culture monsters more than vampires, demons, and so-on. In fiction, by and large, people have some idea what sort of powers and abilities a vampire possesses. Being intelligent monsters, the idea a vampire surviving unseen is plausible. This is less so with a zombie. You must establish how familiar the people of your book are with the ravenous undead.
In The Walking Dead, George Romero's movies do not exist, and they never use the Z-word because the rising dead are a completely unknown quality until that time. In Thom Brannan and D.L. Snell's Dog series, zombies were created in direct homage of the creatures from fiction. Do people know "rules" of killing zombies or is it a completely inexplicable phenomenon? There's nothing preventing you from fiddling around with the concept either.
Alan Wake has the Taken, who are zombies with the serial numbers filed off. The Taken have their souls and minds removed while being re-animated by the black material of the Lake. A horrific fungus might re-animate humans as a monster like in The Last of Us. Mass Effect deals with humans who have been re-animated with nanomachines as cybernetic-corpses. Even changing small bits around can help your work feel fresh and new. It will also leave your reader wondering what sort of "rules" are being followed.
Finally, the important thing to remember with your zombie novel is figuring out what sort of ending you want to go with. Once you have determined what your zombies are supposed to represent, how much damage they've done to the protagonists, and so on--you must figure out where the story will carry you. In Zombie Apocalypse scenarios, the ending is rarely anything but bittersweet. After all, it is rare for the humans to totally defeat the epidemic. More often, it is simply a matter of surviving to the next day.
The genre is also famous for its endings where the entire cast is killed, though this is rare now and could anger readers. In short, my recommendation is to just go with what feels authentic versus what feels happy.
I hope you've found my advice informative. If not, well, see if I help you when Z-Day happens.
Follow Up Article: Zombies vs. Infected