The Ugly Truth About Writing is...some authors only write one book.
I don't mean one book in quantity, although plenty of authors do that. I mean one book in essence.
A few secular examples: Barbara Cartland (fragile, penniless waif with heart-shaped face and huge eyes is rescued by brawny, impeccably-dressed millionaire), Piers Anthony (coming of age story for a boy or girl whatever), and Anne McCaffrey (young person with musical ability and some extraordinary talent must beat the odds to become the best dragon-rider, crystal singer, psionic, you fill in the blank). All three are prolific authors who write basically one book with variations.
Don't get me wrong. I have and love various books by each of these authors. Maybe not Barbara Cartland so much, but her books are like popcorn. Finished in an hour or two and you can still eat dinner. And, I think I own more books by Anne McCaffrey than any other author I read. She's certainly one of my biggest influences. I owe my tendency to link my worlds to her. I believe every book she's ever written involves the same universe in different chronology.
Even Barbara Hambly, one of my favorite authors, writes basically the same book. A stick-like warrior woman and a gangly wizard who doesn't actually use his magic fight some kind of all-powerful evil thing using sheer stubbornness and plodding methodology. I like her because she uses words I don't know and every once in a while, usually once per book, writes a sentence that grabs my heart and sticks in my brain.
Let's examine C.S. Lewis's Narnia series. Those books are nothing alike, except that they take place, for the most part, in Narnia and Aslan shows up in all of them. I cannot say I like all the Narnia books equally, but that is not a bad thing, either. Lewis wrote more than one book, in essence as well as quantity.
And me? I want to think my books will be different. Now that I've started book 3 in a completely different series and genre than the other two, I'm not so sure. Two out of three begin with loners (although loners for different reasons), two out of three include tall, black, bald women warriors (I have no idea how that happened unless I blame Grace Jones in Conan the Destroyer), three out of three feature a mentor, and three out of three involve abandonment issues by parents or guardians. To quote Malcolm Reynolds from Firefly: "Huh."
On the other hand, if I can sell as many books as any of the authors I've listed, I'm totally okay with writing only one book. As long as it's a book people want to read.