Read him. Trust me it'll be worth it. Here's a novelette Isobel Avens Returns to Stepney in the Spring to start off with. The best thing about M. John Harrison is that everytime you pick up another book by him, you are only sure its the same author by the adroitness of the writing. M. John Harrison handles different styles in a way that speaks of his mastery as a writer.
I'm currently reading his collection, Viriconium, which I would highly recommend to anyone. But first read his novelette. It speaks of his style and his mastery. And don't forget to read the afterward and see why he thinks we live in the "Age of the Wanker".