Philip K. Dick was good at marrying lively scifi with thought-provoking philosophical themes, as in Do Androids Dream Of Electric Sheep for example. Dick was also good at frying his brain with drugs and using his experiences in his later writing, like VALIS, the semi-autobiographical novel that processes Dick's very real encounter with what he believed to be a sort-off divine entity. In 1974, he was struck by a wisdom-filled pink beam, began having hallucinations, and started writing an 8000+ page exegesis, a sort of journal about his newfound insights about the world.
It seems that part of him knew that all of that was madness. In VALIS he creates a story that includes himself as a character as well as an alter-ego called Horselover Fat who experiences the exact same things that Dick did in real-life. And that is the setup for what feels like Dick's journey of self-reflection where fiction and the author's real life constantly blur together.

VALIS is a fully harebrained experience. It's light on actual plot but spends lots of time letting the character(s) discuss all their out-of-this-world ideas and wisdoms. It combines so many religious, spiritual, philosophical, and technological ideas into chaotic ramblings, page after page, that it can feel like the book is frying the reader's brain just like the pink beam did to main character Horselover Fat. It might be a complete mess of overwhelming information, but that might just be the point and it does end up being rather captivating. It's challenging but also funny at points, and while it throws so many difficult and crazed concepts at you about God and the world, Dick seems to be aware of how all of it sounds and makes that a point as well.
I don't know if VALIS is very good as a novel in itself, but it is absolutely fascinating as the result of the author processing his own strange experiences that took over his life in a way.