‘I was into Sue Grafton’s A is for Alibi series at the time,’ Charlie reminisced aloud. ‘I really resented having to read anything I hadn’t chosen myself – bit inconvenient when you’re studying English Lit! – and I raced through Beloved without paying too much attention because I couldn’t wait to start D is for Deadbeat or whichever one was next. I remember that the heroine was a woman called Sethe. She killed her baby daughter to save her from a life of being a slave. She loved her so much, she couldn’t bear the thought of her going through the suffering and horrors that she – the mother – had been through. Oh – the dedication was memorably chilling. It wasn’t “To” or “For” anyone, like normal dedications, it just said, “Sixty million and more”, meaning all the black Americans murdered by slavery.’

‘It has to mean something that Billy – or someone – sent that book to Halliday, that particular novel. Why Beloved?’

‘The answer to that question’s only relevant if Billy sent it.’

‘Which we’re officially 80 per cent certain he did, and probably more like 99 per cent certain unofficially,’ Simon reminded her. He dug his thumbnail into the pad of his index finger, turning it from pink to white. ‘It makes no sense. He’s already killed four times. If he wants to kill Sondra Halliday, why not do it? Why’s that morally wrong when killing the others isn’t?’

‘If I had to guess? Sondra Halliday hasn’t got a best friend, so in his eyes she doesn’t deserve it.’

‘Having a best friend isn’t a capital offence.’

‘Simon, he’s a serial killer, not the Archbishop of Canterbury. Maybe he’s morally confused, to put it politely.’

‘Letter doesn’t sound confused,’ Simon muttered.

‘I followed Liv yesterday.’ Charlie half hoped he wouldn’t be listening.

‘Followed her where?’

Damn. ‘To Cambridge.’

‘You mean … followed her without her knowing?’

‘Yup. A proper undercover surveillance operation.’ Charlie was tempted to take her voice to a falsely upbeat pitch: a defence against the frown lines she knew were etching themselves more deeply into Simon’s brow with each word she uttered. She resisted the temptation, and spoke normally. ‘We met for coffee in London. Liv suggested the venue: a place called Drink, Shop & Do – about fourteen footsteps from King’s Cross station. That’s not her neck of the woods and the café was everything she’d normally hate, so I had a feeling she’d be catching a train from King’s Cross after she’d said goodbye to me. I asked her if she was, and she denied it – said she’d chosen somewhere near King’s Cross because she knew that was where I’d be coming in to. Seemingly she’d forgotten she’d never done that ever before, even though I’ve been catching trains from Spilling to King’s Cross for many years—’