Flow My Tears/Now Wait For Last Year
I'm a massive PKD fan, have collected most of his books. Can't wait to see what Linklater does with Scanner Darkly - it's a tough book to bring to film, but hell, they were able to make Blade Runner out of Androids.
The masterpieces of course are Ubik, Scanner Darkly, Three Stigmata and Man In The High Castle. As an earlier poster noted, Counter Clock World is great.
All made when he was wacked out on massive amounts of speed.
I would recommend Flow My Tears The Policeman Said, Now Wait For Last Year, Game Players of Titan and Dr. Bloodmoney as other great books of his to read.
Not a fan of Valis or Divine Invasions - to much metaphysics.
Don't forget about his short stories. Minority Report, Total Recall, Screamers were made based on his short stories.