Philip K Dick rewired my brain when I was a mere lad, after I plucked Clans of the Alphane Moon at random from a shelf in my local library. This was in the 1980s: PKD had not yet become a multi-million dollar industry and his best endorsements came from counterculture figures such as Timothy Leary or fellow denizens of the SF ghetto such as Michael Moorcock.

It was exciting to be a PKD reader back then. Lots of secondary material was being published, such as Paul Williams's interview book Only Apparently Real, or Lawrence Sutin's excellent biography Divine Invasions. Soon it was obvious that not only were PKD's books – with their combination of metaphysical speculation, social satire, bad relationships, and fantastic ideas tossed out as mere afterthoughts – bizarre and wonderful, but that Dick the man was Seriously Weird.

Sure, there was the paranoia, his prodigious appetite for amphetamines, his obsession with Linda Ronstadt and his fear that either the Black Panthers or FBI had raided his house – enough eccentricity for any lifetime, you might think. But that was all eclipsed by what happened on 20 February 1974, when a pink laser beam filled his mind with arcane and beneficial knowledge.

Where had it come from? God? Aliens? A healthy vitamin solution he had quaffed hours earlier? Dick loved to speculate, so much so that this event inspired not only his late "VALIS Trilogy" but also a private work he called The Exegesis. When he died in 1982 it ran to approximately 8,000 pages of analysis, hypothesis and self-questioning.

For some, the pink laser beam is mere lunacy. I recall a TV documentary in which Brian Aldiss dismissed it as the result of neurochemistry gone awry. Others have argued that it was temporal lobe epilepsy. For still others, an unsavoury whiff of L Ron Hubbard hangs over the event. After all, Dick was heavily into theology. Was he starting a cult? If not, would his fans do it for him?

Probably not: Dick's approach to 2-3-74 (as he called the experience, since the cosmic mind invasion was most intense between February and March) was not dogmatic but critical, and he was the first to suggest that it might have been a neurological event. But then again, the light had diagnosed a potentially critical illness in his son which doctors had missed, and he had received information in dream states in dead languages he could not speak. "It" knew things he did not. So what was it?

Dick never intended The Exegesis for publication, and aside from In Pursuit of VALIS, a tiny selection of extracts from the book that was brought out in 1991, it has remained a thing of legend only. Until last month, however, when Houghton Mifflin Harcourt brought out a huge 900-page volume, co-edited by Jonathan Lethem and Pamela Jackson. It's still only about one tenth of the whole thing, but it's a start. But what, if anything, does this text have to offer people who are not Philip K Dick?

Afraid that the answer might be "not much" I started in on it immediately lest it sit on my shelf unread for 20 years like In Pursuit of VALIS. The first thing I noticed is that Lethem et al assume that anyone reading this book already knows what it is, and will only come to it after deep immersion in PKD's fiction. And indeed, Dick himself begins with a discussion of 2-3-74 through the prism of his novel Ubik, where many of the characters are dead bodies lying in "cold-pac", while their ex-employer Glen Runciter seeks to communicate with them from the world of the living … maybe. Was the pink laser beam likewise an invasion of a dead world by something alive?

That PKD had published Ubik four years earlier was not a problem; he writes as if his book might still have related the truth behind appearances. But Ubik doesn't work, as the world is not visibly rotting around him as it was in the novel. However, Dick immediately conceives of another possibility, and I can't help but wonder what his friend Claudia Bush thought when she received a letter in which Dick speculates that a dead bishop named Jim Pike was invading his mind, before suddenly switching to the theory that it might be an ancient Greek named Asklepios. Asklepios's ignorance of Christ suggests something else: did the world go wrong around 2,000 years ago? Is the goal of this higher intelligence to restore man to a pre-Christian path?

A few pages later, however, and Dick confides in Ursula Le Guin that it's the prophet Elijah. Or at least that's what Thomas M Disch (a great SF writer of the 60s and 70s) had suggested. But there is precisely zero possibility that Disch was serious – his take on PKD was that the great man liked to play with his own mental illness. Disch always kept an ironic distance – which is something I miss in the ultra-reverential contemporary introductions to Dick's work written by fanboys with PhDs and MFAs.

These ideas rush past and are discarded within the first 40 pages or so. John Denver also pops up. The Exegesis is dizzying, bewildering, exhilarating, and more or less as strange as it sounds. But again, should you read it? It doesn't contain the answers to all things; it doesn't even contain the answer to what happened to Dick.

Lethem suggests that the reader must simply "surrender". I suspect he's right – but that won't work unless you've read at least 16 of Dick's novels, plus his biography, and love metaphysics. At that point, The Exegesis will bring you extraordinarily close to his unique mind, with its mixture of doubt, wild invention, minuscule detail, grandiose theory and wry humour. Reality collapses and is then remade, over and over again – but what is real?

Who cares? That's part of the game.

In short: if you want to know what it's like to have your world dissolve, and then try to rebuild it while suffering mental invasions from God, Asklepios or whomever, you should read The Exegesis. Then again, you could always try one of Dick's novels, like Ubik, or The Three Stigmata of Palmer Eldritch, or even a minor book such as Galactic Pot-Healer. That one's a lot of fun – and considerably less of an investment of time and energy.