‘This morning I decided to review all the paperwork – the archive of our ongoing shameful failure to catch Mr Dead Mates. I spotted something that didn’t tally: the alibis of Isobel Sturridge, sister of Liam Sturridge.’

Isobel Sturridge. Sam let the name sink in. Once he had, he could think of no possible motive for her to commit Billy’s five murders. For the desire to murder Kim Tribbeck, maybe; Isobel could have been a jealous, overly possessive sister. But Kim was still alive. What could Isobel have had against Linzi Birrell, Rhian Douglas and the others?

‘Isobel Sturridge can’t have been where she says she was when the first four murders were committed,’ said Proust. ‘She told Sellers she was at work – at Rudolphy’s, Silsford’s famous bookshop.’

So famous that Sam had never heard of it. He felt immediately guilty. He and his wife Kate bought all their books from Amazon. They kept agreeing that they shouldn’t, but it was so much easier than making time to go to a physical shop.

‘Do you know why she can’t have been there on those dates?’

Sam shook his head.

‘I used to go to Rudolphy’s as a boy,’ Proust said. ‘In those days, it occupied three different buildings on Guggle Lane, Silsford’s only remaining cobbled street. Two were side by side, with a door connecting them that was sometimes locked and sometimes not. The third was across the street. Two of the three buildings were unmanned – no till, no staff presence at all. Just rooms full of books, new and second-hand, piles of them everywhere. There was a noisy till in the corner of one room that looked and sounded like an old typewriter. Someone would always be behind it to sell books, but if you wanted something from Rudolphy’s mark two or three, you had to take it next door or over the road to buy it. Imagine that these days!’

Sam waited. This was the most personal thing Proust had ever said to him and it made him feel uneasy.