Nicholas Blake's A Penknife in My Heart, first published in 1958, is highly readable, but in some ways a curious book. It's very well-written on the whole, as you would expect from this author, yet there's something slightly amateurish about the way he jumps from one viewpoint to another in a single scene, and in the way he tends to "tell" rather than "show". I suspect this may have been because he did not put as much effort into his novel writing as he did into his poetry published under his real name, Cecil Day Lewis.
Another oddity is that the central situation, of an exchange of murders, replicates that of Strangers on a Train. By the time Blake's book came out, Patricia Highsmith's classic was several years old, and had been successfully filmed by Hitchcock. Yet Blake insists he was unaware of this, and is clearly embarrassed by the coincidence that he also used two of the same character names that appear in Highsmith's story. In a preface, he thanks Highsmith for "being so charmingly sympathetic".
Some may think that it beggars belief that Blake was unaware of the earlier book. I am happy to take him at his word, even though it may be that some information about the film, if not the book, had seeped into his subconscious. It's common for different writers to come up with much the same idea, quite separately. And it's also the case that Blake's story develops in a rather different way from Highsmith's. Some commentators prefer Blake's book, but I think Highsmith's stands the test of time better.
All that said, I did enjoy this story. It's a good, fast read, and I devoured it in a single sitting. It's interesting that Simenon is name-checked in the story; he clearly influenced some of Blake's post-war fiction. Since Blake's book was published, several novels, by authors as diverse as Evelyn Berckman, Sheila Radley, and Peter Swanson, have used the exchange of murders concept in a variety of ways. And it's a concept with rich potential. One of these days, I'm tempted to have a go myself...