If you ask agents and editors what single quality draws them to a manuscript by a new author, the majority of them, at least in my experience, will say “voice.” A strong writer’s literary voice is hard to describe. It may or may not be hard to imitate—but a work that does imitate an established writer’s voice immediately gets branded as derivative or mere pastiche or parody.
The singer’s voice makes a good analogy to writer’s voice. To start by saying what it’s not, I’m not talking about the easily discerned difference between Renee Fleming singing opera and Mariah Carey belting out a pop song. Opera and pop are different musical forms, just as a poem and a novel are different literary forms, and the singers or writers present each in an appropriate artistic style. Voice is more like this: you’re sitting in a greasy spoon in Wichita drinking coffee, and the radio is set to an oldies station. A phrase of vocal music floats past your ears, and you think, “Judy Garland!” or “Louis Armstrong!” Garland and Armstrong are both decades dead, but millions of people still recognize each of these great singers’ unique voice whenever they hear it.
When it’s in the first person, the reader may think of it as the character’s voice rather than the author’s. One of the great challenges to the writer is to shift voice when writing different characters. Some writers do it better than others. To use examples from mystery fiction, Charlaine Harris does it masterfully with the protagonists of her three series, Sookie Stackhouse, Lily Bard, and Harper Connolly. Ruth Rendell, after writing the Inspector Wexford books for years, took the pseudonym of Barbara Vine, in my opinion, less to write in a different subgenre than to write in a different voice. Robert Parker’s voice, on the other hand, is strong and unmistakable, but he doesn’t change it well, so that the Sunny Randall books sound—to my ear, at least—exactly like the Spenser books. Stuart Woods is similarly monolithic: the Holly Barker books sound exactly like the Stone Barrington books, a problem the author solved by getting his two protagonists together and into bed.
Voice at its best is both powerful and memorable. The three examples below came to mind immediately, although I haven’t reread any of these books in years. Mark Twain’s The Adventures of Huckleberry Finn appeared in 1885. Jane Austen’s Pride and Prejudice was published more than 70 years earlier, in 1813. Dodie Smith’s I Capture the Castle came out in 1948. Like Judy Garland and Louis Armstrong, Huck Finn and Elizabeth Bennett and Cassandra Mortmain are unforgettable once you’ve heard them speak.
You don't know about me without you have read a book by the name of The Adventures of Tom Sawyer; but that ain't no matter. That book was made by Mr. Mark Twain, and he told the truth, mainly. There was things which he stretched, but mainly he told the truth. That is nothing. I never seen anybody but lied one time or another, without it was Aunt Polly, or the widow, or maybe Mary. Aunt Polly -- Tom's Aunt Polly, she is -- and Mary, and the Widow Douglas is all told about in that book, which is mostly a true book, with some stretchers, as I said before.
It is a truth universally acknowledged, that a single man in possession of a good fortune must be in want of a wife.
However little known the feelings or views of such a man may be on his first entering a neighbourhood, this truth is so well fixed in the minds of the surrounding families, that he is considered as the rightful property of some one or other of their daughters.
"My dear Mr. Bennet," said his lady to him one day, "have you heard that Netherfield Park is let at last?"
I write this sitting in the kitchen sink. That is, my feet are in it; the rest of me is on the draining-board, which I have padded with our dog’s blanket and the tea-cosy. I can’t say that I am really comfortable, and there is a depressing smell of carbolic soap, but this is the only part of the kitchen where there is any daylight left. And I have found that sitting in a place where you have never sat before can be inspiring—I wrote my very best poem while sitting on the hen-house. Though even that isn’t a very good poem. I have decided that my poetry is so bad that I mustn’t write any more of it.