I've had several conversations lately with others at Drake Valley Press about authors who want to write in multiple genres - and of course, be successful in each.
An old point of view is that an author should select one and only one genre and stick with that. The more modern point of view is that a good author can write in multiple genres. But to be successful, there is a right way to go about it.
I observed an author last year who wrote in multiple genres and could not figure out why she wasn't selling more books. An analysis revealed that the audience for her children's books were vastly different than the audience for her non-fiction and both were vastly different than her erotica. Yes, readers often have eclectic tastes but I'd venture to say the readers who devour the erotica aren't automatically going to rush out and buy her books for their two-year-olds.
It all boils down to one thing: the author's target market.
Only the author truly knows who their fans are; who they are writing for; their image of their readers. And even if you're switching genres, you have to keep in mind who your audience is already and seek to write something that will interest them.
For example, several years ago we had a very successful suspense author who wanted to write an historical book. The subject was textbook material. Had she written it as a textbook, that genre would have started from ground zero; we could not automatically assume her suspense readers would automatically want her textbook.
She opted instead to write the historical book as suspense. It was still factual; but by getting into the head of the main character and writing her adventure as suspenseful as she'd written her other books, it ensured that her fans would follow along. They did - and she broadened her audience by also appealing to fans of history. As a result, her historical book is still her bestselling book, even years later.
The author needs to pay attention to who their fans are and seek to keep those fans even while broadening their fanbase.
We often hear authors lament that their publishers (whether large or small) need to know more about their fanbase than the author does. This always piques my curiosity. We may be working with dozens of authors at once; some publishers work with hundreds over the span of each year. Yet the author only has one person to track: themselves. While we can identify a broad market - romance readers, suspense readers, adventure readers, etc., if the author can't drill down to their fans, it means they are not connecting with them. That connection could be through personal appearances, through blogs and comments, through social networking, or any number of ways.
When writing in multiple genres, the author first needs to identify who is purchasing the books they've already had published. Then they have to ask themselves how they can write the new genre in such a way that it will appeal to their established fanbase.
If the genre is so vastly different that it would not appeal to their current fanbase, they might as well publish it under a completely different name, because it's the same thing as starting completely over with the very first published book.