We get this question all the time:
WHAT THE HELL IS THE GODDAMNED READING ORDER FOR GODKILLER??

Ok, our bad, the reading order is kind of a headache. 
See... Godkiller started as a self-published, print-on-demand comic in 2008 sold only at comic cons and horror cons. And then the animation started before the comic even went into comic shops... so its release has been all over the place.

The main confusion is the 'BOOKS' vs 'VOLUMES.' Trade paperbacks are a pain to print, so first we printed the 'Walk Among Us pt. 1' trade and then after that we just went straight to the omnibus for the complete 'Walk Among Us' and called that 'Book 1.' But fans wanted a slim trade to match 'Walk Among Us pt. 1' so to print that it started alternative numbering systems (which wound up being 'Books' are the hardcovers and 'Volumes' for the paperbacks). 

ON TOP OF THAT, Tomorrow's Ashes + Spiderland + For Those I Love I Will Sacrifice were written to be a single 'Book 2' but they're just too big and unwieldy to print that way, so they were split into paperbacks and hardcovers. Oy vey!

Add to that animations, novellas, audiobooks, vinyl records (!), and extended universe comics and jeez this is a lot to keep track of.

Anyhow, tl;dr -- here's THE OFFICIAL GODKILLER MYTHOS READING ORDER!

1. GODKILLER: WALK AMONG US
You can get the story in these ways:
- Godkiller: Walk Among Us, single issues 1-8, or
- Godkiller: Walk Among Us, trade paperbacks vol 1 & vol 2, or
- Godkiller: Walk Among Us [Deluxe Complete Collection] paperback or hardcover, or
- Godkiller: Walk Among Us, animated episodes 1-3 (each episode covers two comics), or
- Godkiller: Walk Among Us [The Illustrated Film]
* the animations don't include the #8 epilogue story (which is in the collected books)
Whew! Okay from there it gets a bit simple...

2. GODKILLER: TOMORROW'S ASHES
You can get the story in these ways:
- Godkiller: Tomorrow's Ashes, single issues 1-3, or
- Godkiller, vol 3: Tomorrow's Ashes [trade paperback], or
- Godkiller, Book 2: Tomorrow's Ashes [hardcover], or
- soon there will also be a narrative-game of Tomorrow's Ashes

3. GODKILLER: SPIDERLAND
You can get the story in these ways:
- Godkiller: Spiderland, single issues 1-3, or
- Godkiller, vol 4: Spiderland [trade paperback], or
- Godkiller, Book 3: Spiderland [hardcover]

4. GODKILLER: FOR THOSE I LOVE I WILL SACRIFICE
You can get the story in these ways:
- Godkiller: For Those I Love I Will Sacrifice, single issues 1-3, or
- Godkiller, vol 5: For Those I Love I Will Sacrifice [trade paperback], or
- Godkiller, Book 4: For Those I Love I Will Sacrifice [hardcover]

5. MOAR GODKILLER COMING SOON

And then there's the...
== EXTENDED UNIVERSE ==

- GODKILLER Novella Series
Prequel series originally released as PDFs and Audiobooks on the original Animated Series DVD.
-- Godkiller: Silent War (pdf / mp3)
-- Godkiller: The Black Temple (pdf/mp3)
-- Godkiller: The Red Shield (pdf/mp3)

- YOUNG TERRORISTS Graphic Novel Series
Young Terrorists takes place during the pre-apocalyptic period of 'Silent War.' Halfpipe has astrally projected herself into the past to track down the Starkiller, and falls in with Sera's crew taking down The Black Temple along the way.
You can get the story in these ways:
- Young Terrorists, graphic novellas #s 1-2,
- Young Terrorists collected edition [paperback or hardcover]

- THE LONG KNIVES Comic Book Series
Also taking place in the pre-apocalyptic period, this origin story for Bones and Nero was originally released digital-exclusive on Graphic.ly.
You can get the story in these way:
- The Long Knives, single issues 1-12 [digital only],
- The Long Knives, hardcover editions are coming soon.

All that said, you can pretty much jump in with any book. They all start with a 'Previously' catch-up section and hit the ground running. Don't sweat the lore, just grab a book and take the plunge!