Jonathan Franzen, in a recent interview well worth reading, said correctly in my opinion that “almost everything a writer has ever read leaves some kind of mark”. Quite right, as was his recognition of the importance of negative influence. Were I in a flippant frame of mind I would answer the “influence” question with reference to James Clavell. He showed that you could sell millions by writing bad historical novels about Japan: eight hundred page tomes without a single well-crafted sentence in them! Other influences of that ilk came from Dan Brown, Jeffrey Archer and a host of other bad writers – “Good God, I must be able to do better than that!” And yes, I know I'll never sell a fraction of what they have.
At the other extreme are the writers who show you the awesome power of the written word, the capacity of paper and ink (and now e-ink too) to expand your capacity to understand what it means to be human. So Dostoyevsky, Kafka, Hardy and others are influences, but not in the sense that the questioner usually means. One of the influences of these greats is to make mortals realise their own limitations. The best you can hope for as a writer is that the quality of your prose has been improved by your exposure to genius.
So to come to what I think the questioner usually means – which books and writers have most directly and positively influenced your own work – the simple answer in my case is George McDonald Fraser, particularly his “Flashman” series of novels. Fraser showed that you could write funny, intelligent, informative fiction about the past. He demonstrated that not every historical novel had to be so bleedin’ po-faced. In fact, some of the challenge in writing my novel was not to be too influenced by the Flashman books. I hope I have avoided the risk of carrying influence too far into imitation: partly through the Japanese setting and partly because Milligan is “lad” to Flashman’s “cad”, a possessor of human failings in relatively normal quantity rather than the extremes of the bully of Tom Brown.