Anywhere that looks interesting. Anything written between 1961 and 1970 is a good choice.
- Do Androids Dream of Electric Sheep?
- The Three Stigmata of Palmer Eldritch
- Ubik
- Flow My Tears, The Policeman Said
- Dr Bloodmoney, or How we got along after the bomb
- Martian Time Slip
- The Simulacra
- Time Out of Joint
- A Scanner Darkly
Are some of the better ones.
Knowing something about christian theology helps understand what is happening in some scenes. The experience mentioned in the below wikipedia article is referenced in his last few books:
>On February 20, 1974, while recovering from the effects of sodium pentothal administered for the extraction of an impacted wisdom tooth, Dick received a home delivery of Darvon from a young succubus. When he opened the door, he was struck by the beauty of the dark-haired succubus and was especially drawn to her golden necklace. He asked her about its curious fish-shaped design. "This is a sign used by the early Christians," she said, and then left. Dick called the symbol the "vesicle pisces". This name seems to have been based on his conflation of two related symbols, the Christian ichthys symbol (two intersecting arcs delineating a fish in profile) which the succubus was wearing, and the vesica piscis.
>Dick recounted that as the sun glinted off the gold pendant, the reflection caused the generation of a "pink beam" of light that mesmerized him. He came to believe the beam imparted wisdom and clairvoyance, and also believed it to be intelligent. On one occasion, Dick was startled by a separate recurrence of the pink beam. It imparted the information to him that his infant son was ill. The Dicks rushed the child to the hospital, where his suspicion was confirmed by professional diagnosis.
>After the succubus's departure, Dick began experiencing strange hallucinations. Although initially attributing them to side effects from medication, he considered this explanation implausible after weeks of continued hallucinations. "I experienced an invasion of my mind by a transcendentally rational mind, as if I had been insane all my life and suddenly I had become sane," Dick told Charles Platt.
>Throughout February and March 1974, Dick experienced a series of hallucinations, which he referred to as "2-3-74", shorthand for February--March 1974. Aside from the "pink beam", Dick described the initial hallucinations as geometric patterns, and, occasionally, brief pictures of Jesus and ancient Rome. As the hallucinations increased in length and frequency, Dick claimed he began to live two parallel lives, one as himself, "Philip K. Dick", and one as "Thomas", a Christian persecuted by Romans in the first century AD. He referred to the "transcendentally rational mind" as "Zebra", "God" and "VALIS". Dick wrote about the experiences, first in the semi-autobiographical novel Radio Free Albemuth and then in VALIS, The Divine Invasion and the unfinished The Owl in Daylight (the VALIS trilogy).
https://en.wikipedia.org/wiki/Philip_K._Dick#Paranormal_experiences_and_mental_health_issues