There is an American television show called Warehouse 13, a supernatural comedy-thriller, featuring the place – a bit like the warehouse at the end of Raiders of the Lost Ark – where dangerous supernatural relics are stored; things such as Alice Liddell's looking glass or Lizzie Borden's compact. The news that Philip K Dick's annotated copy of the New English Bible, in good condition, with a couple of holograph sheets inlaid, is up for sale on eBay almost inspires one to think that the show's producers have missed a trick – except that this relic, and the unusual circumstances around it, are real.

For those few people who know nothing of him, Philip Kindred Dick was a prolific science fiction writer and mystic who died in 1982; many of his short stories and novels have been filmed, one or two of them memorably: Blade Runner is an adaptation of his Do Androids Dream of Electric Sheep? and Richard Linklater made a powerful animated version of Dick's novel about drugs and law enforcement, A Scanner Darkly. In 1974, Dick had the first of a sequence of visions, which raise some fascinating questions about the nature – and the value – of religious experience.

It is the nature of religious and mystical experience to be hard to put into words – if Dick did so more effectively than many, it is precisely because he was a pulp writer of genius. His sense, for example, that we still live in the "iron prison house" of a materialistic Roman Empire and that, in some very real sense, it is always 50AD – or his attempt to describe God as a Vast Active Living Intelligent System, an artificial intelligence orbiting another star, were partly religious insights, partly craziness and partly plot ideas. It is not to be reductive about religion to accept that these three categories have a certain amount in common.

Dick is one of the great artists of paranoia – he took a lot of amphetamines to keep up his frenetic rate of literary productivity before discovering that he did not need them. For years he claimed to have been burgled and spied on, before the revelation of Richard Nixon's enemies list made it rather likely that he was telling the truth. His work features conspiracies; revelations of underlying truths; mental patients forming a viable society based on diagnostic caste divisions; an alternate world in which the axis won and a version of our world is produced by a novelist using the I Ching for divination; and androids passing themselves off as human but revealed by their lack of empathy and compassion. There is a sense – of course there is a sense – in which the particular direction taken by the deepening of his religious convictions is of a piece with some of his other ideas.

There is the sense of being special – believing yourself to be in mental touch with a community of persecuted Christians in Judaea who help you organise your tax affairs is probably more fun than just going to church. There is the sense of knowing what other people do not know – eternal Rome was a more glamorous hypothesis than four more years of Nixon. There was the sense of significance – a Philip K Dick whose decades of producing paperback fiction for a quick buck were a preparation for his receiving the word of God was a man who could respect himself more. (After all, he was not to know that, after his death, he would be a dominant figure in popular culture whose fiction has received more critical attention than most of his colleagues put together.)

Dick received the first instalment of his vision along with a packet of painkillers and in a sense religion was always about healing his pain, but not that alone. He knew, and was fearfully impressed by, the heretic Episcopalian bishop James Pike and his last novel The Transmigration of Timothy Archer, which deals with Pike and a version of his death on a spiritual quest in the desert, is one of his best and most humane. Dick always had a problem with women – his several divorces, his obsession with Linda Ronstadt, his many female villains – and it is touching that this last book has, in its narrator Angel, one of his best female characters.

In the end, those of us who are agnostic find it easiest to accept the existential validity of religious experiences that make those who endure or enjoy them better writers and more likable human beings. I doubt that the few notes Dick scrawled in his copy of the Bible are more useful in their insights than his own books, especially that last one.