I finished reading Darker Than You Think one night last week on the bus home from work. Gollancz reprinted it in 2003 as number 38 in their “Fantasy Masterworks” series. I'm not entirely sure if it truly qualifies as a masterwork or not, but it's definitely one of the best books I've read recently.
I haven't delved deeply into the realm of werewolf fiction, but I'm fairly certain this isn't a typical example of that sub-genre within horror or dark fantasy. It also doesn't neatly fit into the urban fantasy genre. While it's true that in the world of the novel, most people don't believe in werewolves, which is a common trait in urban fantasy, this novel doesn't really present werewolves as strictly supernatural.
Just like the vampires in Poppy Z. Brite’s Lost Souls, the werewolves in Darker than You Think are depicted as a separate species of intelligent life living among us regular humans.
At least that's one perspective presented in the novel. Another is that the supposed lycanthropy is actually one man's descent into paranoia and depression.
The main character, Will Barbee, isn't sure which to believe, and Williamson does an outstanding job of keeping the reader equally uncertain. Given the amount of sf/horror/dark fantasy on my reading list, I was inclined to believe a supernatural explanation and more than willing to accept an alternate species, but I was never quite sure where the novel was headed.
Overall, I thought Darker Than You Think was excellent. It's well worth reading, even 60+ years after its original book publication (70+ years after the publication of the magazine version in Unknown Fantasy Fiction, December 1940).
For some reason, I haven't read much (if any) Williamson before this. I find that somewhat surprising since his work has been around for a long time (his first story was published in 1928 and his last novel in 2005). He wrote in genres that I tend to read extensively, namely Science Fiction and Fantasy, and he was highly respected in both genres. In 1976, he was the second author named a Grand Master of Science Fiction by the SFWA, and in 1994, he received a World Fantasy Award for Lifetime Achievement.
I hope Darker Than You Think isn't atypical of his work as I'm looking forward to reading more of his books.