I once read an interview with a well known author in our field. She was explaining her career choices in the context of some seasoned advice from her agent, who was explaining what she was doing wrong. To this day I consider it one of the most depressing writer interviews I’ve ever read. Not because of the author, who is obviously doing well and is comfortable with who she is and what she’s doing.
No, it was because of the agent’s advice. The author had written a fairly well-reviewed first novel that had sold okay but not great. Then she wrote another fantasy novel, but not much like the first one. And then she did something else for her third. See the trend? So her agent sat her down and said something to the effect that “When you write books that have no relation to each other, each one is like a first novel and you’re starting from scratch every time. It doesn’t work.” The point being that the author was doing many different kinds of books and this was a bad thing. Readers who liked her first book wouldn’t necessarily like her second, because it might not be about what interested them as a reader. This approach was preventing the author from building a readership, and if she wanted a readership (and thus a career) she was going to have to pick one kind of novel and stick to it. And one kind doesn’t mean “fantasy” as opposed to sf or horror; all her novels up until then were fantasy. Just not consistently the same sort of fantasy. That was what had to change, and a consistent approach (traditional fantasy adventure, urban fantasy, steampunk, whatever) chosen. Otherwise she would never be able to establish her “brand” as a writer. The author took her agent’s advice and she did change her approach and it worked wonders for her career.
In other words, in terms of branding, career and audience building, the agent was exactly right.
For those of us who still cling to the idea of being able to write one-off, quirky individual books and still have a career, that is depressing. Though I guess the fact is that one can very easily have a career that way. Just so long as you don’t expect to make a living.
Richard Parks 
That can work, depending on the kind of writer we’re talking about. A writer who has a distinctive voice that comes through whatever they write can do well. Then there are those of us who write in the voice of the story itself, and that can change dramatically depending on what we’re writing. In short, it depends on whether you’re a fox or a hedgehog. (“The fox knows many things. The Hedgehog knows only ONE thing, but knows it very, very well.”)
It’s probable that there had to be *some* unifying element in the author’s three disparate books. The books’ tones might be similar, or the sentence-building style, or perhaps there are common themes behind the plot. An author puts pieces of him/herself in everything he/she writes. A pattern’s bound to emerge somewhere — and if one of those patterns is good writing, I see no good reason why the author shouldn’t do just fine.