E pistolary novels were once all the rage, from the epic Clarissa to the lurid fun of Dracula. They don’t come along very often now, perhaps because they can be tricky to do well: all those gaps and omissions, the need for a flawless command of tone and voice, the problem of creating movement within an unusually hermetic form. But every now and then a book appears that’s a breakout success. The 2000s saw two epistolary smash hits in We Need to Talk about Kevin and The Guernsey Literary and Potato Peel Pie Society (boy, are those different reading experiences), while in the 2010s there was Where’d You Go, Bernadette? Now we have Virginia Evans’s The Correspondent. It’s been one of those word-of-mouth sensations that puts a spring back into publishers’ steps, a bestseller on both sides of the Atlantic, now shortlisted for the Women’s prize for fiction . It’s easy to see why, given that it’s such an immensely enjoyable read.…