This idea grew out of a chance conversation with my friend Graham Coster, publisher of Safe Haven Books.  I’d been reading Janet Malcolm’s fabulous The Silent Woman, about her exploration of the relationship between Sylvia Plath and Ted Hughes, in which she describes writing a letter to a fellow biographer – and then not sending it.  Her analysis of the motives that made her write, but not send, the letter is fascinating: the unsent letter, Janet Malcolm suggests, would be an interesting genre for study.  In fact, Graham said to me, it would make a good book. And then he said: ‘When are you going to write it?’

So I did – with his help in researching and sourcing letters from history, literature, diaries and elsewhere.  Letters written in anger and then thought better of, letters that no longer needed to be sent because writing them was therapy enough, and letters that remained undelivered because of mishap, misdirection or some other intervention. Plus letters that were expected but never arrived (and even some that did arrive, but were then treated by the recipient as though they hadn’t…).  It was a chance to revisit favourite books – everything from Iris Murdoch and Dorothy L. Sayers to Anthony Buckeridge’s school stories (the unsent drafts of Jennings’ and Darbishire’s first postcards home are an enduring delight) – and to investigate the circumstances that left sometimes quite significant letters quietly, poignantly, unsent.

The title, incidentally, comes from a letter of Emily Dickinson’s (later turned into a poem): her point was that letters are like unexploded bombs, carrying material that can wreak havoc in our lives, so we should be careful what we write.

What a Hazard a Letter is: The Strange Destiny of the Unsent Letter is published by Safe Haven Books on 20 September, price £14.99.