SPIDER<i> By Patrick McGrath (Poseidon Press: $18.95; 204 pp.) </i>

“Spider” may be a novel about mental illness that masquerades as a thriller. Or it may be a thriller whose impact is blurred by the derangement of its narrator. Either way, it’s a suspenseful and evocative piece of mood writing that disappoints a little at the end.

The narrator, Dennis (Spider) Cleg, grows up terrified of his father, a plumber and weekend gardener in the East End slums of London. In 1957, released from an insane asylum to a boarding house in his old neighborhood, he broods over scenes of fog, grime and desolation that revive 20-year-old memories of how his father killed his mother, buried her in a potato patch and took up with a prostitute whose last name matches that of Spider’s current landlady.

Spider himself has a near-namesake: the kidnaper Clegg in John Fowles’ “The Collector.” Both keep voluminous journals. But whereas Fowles’ language reflects the twists and limitations of Clegg’s mind, Patrick McGrath (whose previous works include “The Grotesque” and “Blood and Water and Other Tales”) insists on making the working-class Spider a polished literary stylist. The narrative glides on, unruffled, even as Spider’s past and present get jumbled together and he sinks back into psychotic delusions--that imps are rampaging in the attic and that his intestines are knotted around his spine.

In the end, Spider thinks he remembers that he murdered the prostitute. There is another, more horrible possibility that McGrath hints at--a possibility that would justify the novel’s slow, atmospheric build-up, but McGrath can only hint. All of Spider’s perceptions are equally suspect, and the prose offers us no way to evaluate them. Deprived of the true thriller shock, we are left with a case history: the account of an unfortunate youth’s descent into a private hell.