‘I’m sorry, detective, but I can’t tell you.’ She looked at the book meaningfully and Patrick smiled, picking it up.

‘I know this book,’ he said. It was The Snapping Point by Dr Samuel Koppler. He had a copy under his bed at his parents’ house.

‘Really? Is this the kind of thing they get you to read when you join the police?’

‘Not exactly.’

She gave him a curious look. ‘Well. I’m sure if you didn’t find it useful before, you will now.’

Patrick laughed to himself as he left Dr Hudson’s house, enjoying her sneaky way of telling him who she had referred Fredericks onto. He had asked Hudson if he could borrow the book, even though he had a copy at home, thinking it would be a useful ice-breaker. In his experience, people who regarded their opinions highly enough to fill a book with them usually responded well to flattery.

Samuel Koppler’s office was located in a large, converted townhouse. Patrick pressed the buzzer and waited, wondering if the doctor would have already gone home for the day. But as he was about to give up, the door opened and a middle-aged woman came out. Patrick caught the door and went through, climbing the stairs to the third floor.

He found the outer door to Koppler’s office and went through. There was nobody behind the reception desk but he could hear classical music coming from behind the door to what he guessed must be the psychiatrist’s consulting room.

He knocked and the door was immediately opened.

‘Yes?’

‘Doctor Samuel Koppler?’ Patrick flashed his warrant card and introduced himself. ‘I was hoping to talk to you about a former patient of yours.’

Koppler turned to look Patrick up and down. The psychiatrist was in his early fifties, almost completely bald, and huge: around six foot four, with a slight stoop. He frowned and tilted his head from side to side, before finally saying, ‘You’d better come in. Though I was just about to leave for the day.’

‘Understood. But it will only take a moment.’

The office was similar to Catherine Hudson’s: dark wood, a couple of comfortable chairs, certificates on the wall. A computer sat on the desk beside a huge stack of papers held in place by a chunky paperweight. There was a distinctive smell in the air too, acrid and smoky, like Koppler had been burning something.

Koppler had spotted the book Patrick was holding, and Patrick followed his gaze to the large letters on the cover.

‘I first just wanted to say, Dr Koppler, that I’ve read The Snapping Point and, well, it was an illuminating read.’

The psychiatrist raised his eyebrows, as if amazed to find that police detectives could read. The frown remained in place but he was visibly pleased.

‘Thank you, er, detective . . .’

‘Patrick Lennon.’