Is there a novelization of Pandorum?
If not, did anyone else write about the inhabitants of a generation ship going feral?
"Orphans of The Sky" by Robert A Heinlein. It is very different from most of his other books and does not seem to connect into his "universe". It has a fast plot with a space opera feel to it.
"Universe" by Robert A Heinlein is a story along these lines.
Like most of RAF's work it is well worth reading.
And in the "Helliconia" trilogy by Aldiss there is a spaceship, where the inhabitants lose contact with earth. Their society degenerates as well.
Hull Zero Three by Greg Bear is quite similar to Pandorum. Many people have remarked on the similarities, which has cause some bad reviews. On the other hand, plenty of good reviews on Amazon too, so it's still on my list.
HULL ZERO THREE is an edge of your seat thrill-ride through the darkest reaches of space, from one of the genre's biggest names. Perfect for fans of Arthur C. Clarke's RAMA or the film EVENT HORIZON. A starship hurtles through the emptiness of space. Its destination - unknown. Its purpose? A mystery. Its history? Lost. Now, one man wakes up. Ripped from a dream of a new home, a new planet and the woman he was meant to love in his arms, he finds himself wet, naked, and freezing to death. The dark halls are full of monsters but trusting other survivors he meets might be the greater danger. All he has are questions: Who is he? Where are they going? What happened to the dream of a new life? What happened to the woman he loved? What happened to Hull 03? All will be answered, if he can survive. Uncover the mystery. Fix the ship. Find a way home.
"Mayflies" by Kevin O'Donnell tells the story of a generation ship, built as an escape from a dying Earth.
Without going into too many spoilers, the central computer fails and shuts off the main drive, making the journey take much longer than expected. The passengers don't handle this well.
I'd add The Book of the Long Sun tetralogy by Gene Wolf.