"Humpty Dumpty In Oakland" was recently published by Tor in hardback -- I believe it may have been available in the UK some time ago. As far as I know it's the last of his mainstream novels to be published -- I've read everything else mentioned on his website. It's set in 1957 (or so), and tells the bleak tale of Al Miller, a youngish man who sells used cars off a small lot, and the trouble he gets into when the man who owns the lot he's renting sells it.
The pleasure of most Dick novels is that the characters are trapped in a world they don't understand, and muddle along as best they can. They seem completely blindsided by everything that happens to them, and indeed sometimes they are surprised by their own actions. "Humpty Dumpty" may not be the best among his mainstream works -- Al Miller is an unsympathetic anti-hero who seems unable to handle any aspect of his life. It's a bit implausible that anyone as clueless as Al could have gotten as far in life as he seems to be at the start of the book.
Be that as it may, there's plenty to like -- the vivid description of the Bay Area of the 50s, the ambiguous race relations of the day, and the occasional flashes of sad farce. I particularly like the description of a Barbershop Quartet record Al is forced to listen to in preparation to (really!) become a scout to discover new Barbershop Quartet groups in California's arid outback:
"...The Sound had penetrated as pure vibration, as pure disturbance of the air. It was sound reduced to cycles per second, reinforced sound emanating from four vocalizing simps who had managed to tune themselves to the exact same pitch, intervals apart, and then this sound had been reinforced by all the modern electronic gadgets, sound chambers and all the rest, so that in the end it bore no relation to what the performers had done, bad as it was. The original sound could have been escaped; but this final product, he realized, would get a person through concrete and sandbags and steel. It would follow him into the bomb-shelter and even the grave."
I'd always thought that Barbershop Quartets were a bit corny for my taste, but it took Dick to show their true horror. That happens frequently in this book, where a character's estrangement from the world around him renders the familiar horrible and frightening, in a way that's mundane, artful, and hilarious all at once.