Then I bought and read his 1995 novel Blood Crazy in its 2001 reprint edition. And while it adolescent-ish overall tone and style, not to mention some patently ludicrous scenes, weren't my normal horror cup of tea -- I'm drawn to literary and philosophical stuff in a weird fiction vein, as in Lovecraft, Ligotti, Machen, Ted Klein, Algernon Blackwood, Laird Barron, (some of) Michael Shea, etc. -- it did eventually end up generating a nice apocalyptic tone. I think there's no doubt that its depiction of an England overtaken by apocalyptic disaster in the form of a zombie-like horde was both an expression and an influencer of thing like the movie 28 Days Later and its ilk.
The premise is that one day every human being over 19 is possessed by a sudden and irresistible urge to kill everybody younger. These adults are transformed into a hive-mind horde of zombies, and the grue and carnage are plentiful. Some young people survive and band together to try and figure out how to live and build a new world, even as the raving adults still try to stamp out the rest of them.
The single most interesting thing I remember about the novel (and this is a spoiler, so be warned) is that Clark built the whole thing around a surprisingly profound seed idea. Specifically, he posits that the monstrous transformation of the world's adults is the result of a danger inherent in the inbuilt division of the human psyche into ego and unconscious. The transformation of adults into young-a-cidal monsters is the action of an unexpected evolutionary development brought on by overcivilization and occurring in the unconscious mind. Basically, adults suddenly mutate into a collective-minded new species and immediately begin seeing their young as a separate species competing for space and resources.
The young, for their part, don't yet have their conscious and unconscious mind shut off from each other as completely as the adults, so the change doesn't "take" in them. But then they have to learn to access the resources of their own unconscious minds in order to compete and survive.
As I think back on the book, I'm reminded of many striking resemblances between what it narrates and Tony's daemon/eidolon dyad. For example, there's a scene in which the young protagonist, Nick, has begun to learn about all of this psychological stuff, and about the way the unconscious mind was deified in ancient human thought, with its "voice" being framed as the voice of a god. He decides to rely on it to see if it will actually help him, and in fact it does, helping to elude murderous adults and accomplish other things, even making leaps of intuitive knowledge by leading him to find items that have been misplaced. Again, Tony's gathered accounts in The Daemon of people finding themselves behaving in ways they haven't consciously chosen, only to find their lives saved or otherwise benefited by these spontaneous behaviors, comes to mind.
I just now found an interview with Simon (available only through the Google cache) in which he talks about his thought process:
When I set out to write "Blood Crazy" I didn't intend to explain the adults' madness; however, as I wrote it something clicked in my head - and all that I'd read about psychology and Jung supplied the answer. It's generally accepted by psychologists as fact that we do have two minds inside our heads. The conscious mind, with which we think, makes decisions about what to eat for lunch, whether or not to watch Ray Cokes on MTV and so on. Then there is a second mysterious mind hidden in the unconscious. I thought this was a fascinating idea and I ended up trying to explore what might happen if this mysterious second mind should rebel against the conscious mind."
Who knows, some of you who like Tony's books might like this novel. Now that I've written about it, I'm possessed (daemonically?) by an urge to reread it with these things in mind. I think it may rank as his most popular book. I know it has a cult following. I find this especially interesting given is daemon-esque premise.