The beginning of the book talks about Dick’s substance abuse, mental health, and a religious vision he had in the early 1970s. This is done through the character Horselover Fat, an alter ego and stand-in for Dick. 
Along the way we learn about the Black Iron Prison, meet a David Bowie-like celebrity called Mother Goose. Most importantly we learn:

THE EMPIRE NEVER ENDED

If you’re in the mood for trippy religious science fiction, then VALIS may be your ticket. The follow-up “The Divine Invasion” is worth a read too!

3.4 Stars: Only the last third is enjoyable to read, but it’s all insane. If this is autobiographical no wonder PKD writes what he does.

?????????

this one was a ride. basically consists of pkd trying to rationalize and figure out the meaning behind his own delusions, and explores this via essentially splitting himself into two characters: horselover fat and phil (although within the context of the story theyre basically the same person). the split allows for pkd to comment on his delusions from an objective, rational standpoint, which leads to an interesting dynamic and lots of funny moments. one thing i loved about this novel is how even though there are all these crazy, albeit extremely interesting, tangents about religion, mysticism, philosophy, etc, its all grounded by the very personal through line about someone trying to rationalize an irrational universe, through the lens of mental illness, grief, and drug abuse. its a very personal novel and has a lot of heart, and although i like pkd's other books, its something that they often lack.

the ambiguous nature of the ending essentially offers us no conclusion; fat continues to look for the savior, he and phil are split again, and they both begin to fall back into the "chinese finger trap". phil gives horselover fat a rational explanation for his delusions, but he continues to look for the savior anyway. this essentially boils valis down to what makes it so interesting: pkd's own acute awareness to his delusions, succumbing to their madness anyways, and how the answers to this batshit universe are always almost in reach, before fading away and crumbling again. its a cycle that pkd was constantly stuck in, and one that he reflects upon in this book.

i tried to read this book a year ago, but i dnf'd it and failed, knowing that it wasnt the right time to pick it up. i actually didnt intend on reading it again so soon; i had been thinking about it one day and then just stumbled upon it at a bookstore by (seemingly) pure chance... is it evidence of the hologrammatic nature of our universe, or just a coincidence......????? probably the latter. im glad i picked it up though, from the dick ive read this is clearly his magnum opus.