Filth is a novel written by Irvine Welsh, released in 1998.
Plot synopsis (teaser - no spoilers)
It is about a less-than-charming Scottish Detective Sergeant, Bruce Robertson. He is an angry young man who is betting high on his upcoming promotion in the Edinburgh police force, and is willing to do whatever it takes to reach that goal..
In between wild bolts of excessive drug abuse, wild sexual adventures (both with prostitutes and pretty much anyone else he can get his hands on), squeezes of alcoholism and a definite set of psycotic streaks, he tries to solve the case of a murder of a prominent figure of the black community.
Of course, being the exact opposite of politically correct, Robertson is deeply racist, and he doesn't do much to solve the case.
About the language
The language in the book is an approximation of Scottish (I assume it is Scottish, in any case), although the dialect becomes more and less apparent over certain sections of the book. I am assuming that the parts where Robertson (who is also the narrator) talks with the broadest dialect are significant in some way, but (without having studied this thoroughly) I couldn't immediately find a pattern.
If your English is on par, the Scottish (although a bit difficult at first) isn't too bad.
About the book
Expecting another Trainspotting, I was disappointed. The book is good, but it lacks the sarcastic touch and the wit prominent in TS. Instead of being a hard black comedy novel, it has become a sinister, dark description of some sad bastard's life, on his path from being an evil police officer to an insane, babbling piece of (almost) human wreckage.
However, despite the fact that Robertson drives himself nuts, I found it very hard to have any sympathy with the guy. Although the ending is surprising, it is also predictable (ah - the contradictions!!), and even though it brings a certain closure to the novel, I found myself very unsatisfied with the result.
I would recommend reading the book, even though if you are Welsh-curious, I'd suggest giving Trainspotting a shot first.