Possibly the most complex of Dick's works it ranks, along with A Scanner Darkly, as his greatest achievement, blurring the boundaries between psychology, literature and science fiction.
After a series of mental traumas and breakdowns Horselover Fat (Dick himself)discovers a cipher hidden in the most banal representations of reality that unlocks the secrets to the universe. Completely confused Fat stumbles from revelation to realisation, constructing complex cosmologies on the way and dicovering the true purpose of the universe and the reasons for all the mistakes.
Along the way he utterly confounds therapists, friends, his own paranoia and the reader, eventaully stumbling on the secret purpose of Valis, the divine operator.
This is a classic rendering of the psychotic mind, the incredible truths it can uncover and the complete confusion and disorientation it suffers along the way.
If you like Dick read this book, it reveals him as fragile, vulnerable and fascinating.
And if you're interested in asking questions about anything, read this book - it won't answer any of them, but it sure as hell will put a different slant on the way you ask them.