Faithful adaptation?

Listened to audio version of the novel yesterday, and no, I would say it is not really faithful.

Largely.

There are minor differences:
* the supermarket is on level 10 (book), 15 (film)
* no mention of horses in book
* I don't think Helen Wilder was pregnant in the book
* in the film, Laing is the last surviving member of his family - his sister is dead before he moved into the apartment
* new characters here and there such as Monroe, Simmons
* in the book, there is no explanation of the first human death: the jeweller falling to the parking lot. In the film, there is a scene where Laing is at work and showing his students an examination of a dead man's head. One of his students, Monroe passes out and lands heavily and medical tests are ordered. Later, Royal invites Laing to an exclusive party in the high rise with a French theme where Monroe mocks him before Laing is ejected by Simmons. Back at medical school, Laing examines a scan of Monroe's brain (I'm not a doctor so I can't tell what that scan result means), Monroe comments that it is interesting that they both live in the same apartment but how he lives in the 39th floor and tells Monroe that there may be a problem. Monroe is concerned that his father won't take it well. It's implied that Laing is responsible for Monroe's suicide.
* In the film, Royal's relationship with his wife has not been physical for 6 months.
* Charlotte Melville's son Toby features more heavily and it is later revealed that he is Royal's illegitimate son.

I think the movie captured the basic concept of the book and also the craziness of it but not much else. The plot of the book is much different.

Mostly. There's a few mino changes that are pretty standard with any movie adaptation. I think Charlotte in the film was a combination of a couple of other characters. A woman who lived above him with her young son and another who he started a relationship with at the end (Along with his sister, who is alive in the books).

The film glosses over a lot of the gradual breakdown in the building and how people begin to embrace their primitive sides.

Toby wasn't Royal's son.

A big part of Wilder's story was him abandoning Helen and his kids early on and slowly climbing to the top floor for most of the book. In the end after he shoots Royal he is brutally stabbed and eaten by the gang of women, but Helen also takes part in his murder.

I'm writing a play. It's a cross between Glee and The Road.