She steered the car to the left to miss a dark lump on the road: brown with patches of red. She looked away quickly to avoid seeing spikes if there were any there to see, and told herself it probably wasn’t a hedgehog. The thought of hedgehogs or elephants being killed bothered her in a way that animal deaths in general didn’t.

Simon didn’t know this about her. She wanted to tell him, but now wasn’t the time. If he wasn’t paying attention to her description of Beloved, which he’d asked about, then the topic of disproportionate sympathy for hedgehogs was unlikely to engage him.

‘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?’