Reasons to like this book:
We're in Edwardian England, chasing anarchists.
There's lots of surreal descriptions of London, including a snowstorm in the City.
A fair bit of humour not to mention strange chases across northern France and London.
Clever twists of plot
A very English view of Christianity and the Devil (somehow in English literature, he always comes across as such an attractive chap)
It's a metaphysical detective story
It's a meditation on good v evil, chaos v order
It rattles along at a fair old pace

This book seems to foreshadow all those Moorcock/Moore/RRankin surreal fantasy that I love so much, so for me it was a real treat.