He's a Scottish writer, generally very highly regarded, who writes about hard-hitting subjects, such as drug abuse. Personally, I donn't like his style all that much - there's so much swearing that ever other word seems to begin with F - & the controversy is that he seems to glamourise drug-taking. His stories are often very surreal (a la The Acid House).
His most famous two are The Acid House & Train Spotting.

I have only read Trainspotting, very recently. Worth the effort. It evokes emotion.
After you finish the book certain passages keep coming back to memory time and again.
It has the ring of authenticity in the writing, like he has really been there and lived it. I will read something else of his when I come across it. That's all I can say.

Welsh is a genious. No doubt about it.
Yes the language will put people off. The dialect, the slang, the swearing, he isn't for everyone. Don't lend his books to your gran unless she's a real tough old bird.
But boy does he know how to get a response from his reader. You'll feel like it were you that grew up on a council estate in Scotland, even if you're from LA and have never seen a poor person other than on TV.
Maribou Stork is one of my favorite books of all time. Such an ambitious plot but he totally nails it.