Tim Powers is a fantasy writer who spins out tales of wild, mystic conspiracy that are so believable and weird, we're lucky he didn't follow L Ron Hubbard's example and found a religion, or we'd all be worshipping in his cult. Along with James Blaylock and KW Jeter, Powers was one of three young, crazy genre writers who served as Philip K Dick's proteges, and Powers gives us a glimpse of where Dick may have ended up if he'd managed to beat his own worst self-destructive impulses.

Powers' latest (and, to my mind, best) novel is Medusa's Web, a meticulously researched novel about an extended, dysfunctional family whose secret history is bound up with "spiders" — eight-legged drawings that allow you to travel out of your body and into other places and times, when other spiders were taken.

The spiders have been with us for centuries; Medusa's eight-snaked head was a spider, and the reality is that looking into her eyes didn't turn men to stone, it merely transfixed them as they imbibed the experiences of thousands of other people, including, possibly, people from other planes of experience. The spiders are alive, two-dimensional beings for whom all times are the same time, and when you notice them, they notice you and take you into their world, where all objects appear infinitely tall, where depth vanishes. Taking spiders means letting strangers through all history enter your body, or sometimes, future versions of yourself, and their use of your limbs and mouth can leave you injured, or even dead.

But the spider-users crave them, because there's that moment, that moment of dislocation when you're in no body and every body, when your self is blissfully obliterated. This is a story about drugs and addiction, in the phildickian style, but Powers far surpasses Dick.

Powers's story is set in Los Angeles, in the huge, decrepit mansion of the old lady who served as matriarch to the family — a house that recalls the House of Usher in more ways than one — a mansion that has played a key role in the history of spiders and Hollywood, involving a cast of characters that includes Rudy Valentino and many of silent film's most debauched leading lights. The Powers method for constructing secret histories is to line up seemingly unrelated historical events that share a place or time and then invent a connection to explain the coincidence, and it works brilliantly here, as the very geography of LA and the history of film are recruited to give a plausible footing to the story Powers tells.

The themes of Medusa's Web have been part of the Powers canon since the start; possession was a major theme of Dinner at Deviant's Palace, and it returns in his World Fantasy Award-winning masterpiece Last Call. The ancient conspiracies stretching back into Greco-Roman mythology will be familiar to those who enjoyed Hide Me Among the Graves as much as I did.

But Powers isn't rehashing his favorite themes, he's honing, making them sharp enough to slice away our reality in that Philip K Dick way. But Dick was a self-destructive drug addict whose speed ran ahead of his prose, producing brilliant but uneven books with stilted dialog and weird protrusions in the plot that remind you that you're reading a story, not a history. Powers went from being "a big fan of alcohol" to a teetotaler more than 20 years ago, and he's got both the intimate knowledge of being out-of-control and the sobriety to be in control when he writes about it. If Dick had gone sober and practiced his craft for 20 years longer, he could have written a book like this.

Reading any Powers novel is a visit to an alternate reality where everything, even the stones, are conspiring with one another. Medusa's Web is a gorgon of a novel, constructing that sinister, paranoid reality so perfectly that it transfixes anyone who dares crack its cover.

Medusa's Web [Tim Powers/William Morrow]