THE MONSTER IN THE BOX, by Ruth Rendell. Scribner; 287 pages; $26.

Inspector Wexford is older now, and, to his dismay, wider, but The Monster in the Box is no lighthearted farewell to a beloved detective.

Wexford is haunted by his very first case, in the middle of Sussex where he is still working. It was a strangling, for which a man was caught and sentenced, but Wexford has never believed they got the right man. He knows who the right man was, and he also has a gut feeling that the same man has strangled several other people years apart.

He first saw the man the night Elsie Carroll was murdered. The man was walking his dog, who was relieving himself against a tree. Most people will look away at policemen doing their grim jobs, but this man just stared at the young Wexford.

And then he nodded. It was the nod that convinced Wexford he was the murderer, that and the smug satisfaction in his face as he watched the police come and go during the investigation of that murder.

The man was named Targo. He loves animals and hates people, has a crab-shaped birthmark on his neck and cheek that he covers up with a procession of scarves.

All this backstory is communicated via long, mezmerizing monologues from Wexford, as he tells a colleague the story. Targo was never prosecuted because there was no evidence. Wexford knows he’s right, but he also knows he can’t prove anything and, in any case, it was all so long ago.

“The man began to take on the aspect of a character in a recurring dream, someone who has no existence in life but only in the dream, where he is vivid enough and haunting enough.”

And then, more than 30 years later, there is another strangling, and Wexford realizes that Targo is back, and very close indeed — taunting Wexford by murdering Wexford’s gardener, just to show that he can.

Rendell backs up this A story, which is totally compelling, with a B story that isn’t anywhere near as interesting. Given her great skill, there’s no doubt that the two stories will converge, but it takes a bit too long, although when the narratives do come together it’s in a surprising way, followed by a lovely surprise and appropriately chilly, major-chord ending.

Ruth Rendell remains among the three or four best mystery novelists alive — procedurals in layers, written through a deep knowledge of character and the endless human capacity for perversity. Wexford has been the protagonist of more than 20 of Rendell’s 50-odd novels. He’s an intellectual of sorts, but he’s a bulldog when it comes to his business.

This dogged, admirable character has the leaping instincts of Holmes combined with the resolution and decency of Watson — an Englishman straight and true, created by a modern master.