As mentioned in many of the comment, fiction books are usually much simpler than technical books. When I've typeset novels, I've normally used the
book class and tweaked it with some new sectioning commands, then called it a day. How much redefining you need to do will depend on what kind of novel you want to write; are we talking about a longer novel, with several parts (say, The Stand), or a typical sci-fi paperback (say, Snow Crash)? Named chapters, or merely numbered? (I typeset one where each chapter was a different date, so the headings here were a bit more complicated.) In the simplest case, you can do something like this:
\vskip3em plus1em minus1em%
\Huge\scshape Chapter \thechapter%
\penalty10000\vskip3em plus1em minus1em%
\vskip2em plus0.5em minus0.5em%
\penalty10000\vskip2em plus0.5em minus0.5em%
This will give you the fiction-looking following (plus a not-very-fictiony TOC at the top, just to show that it works):
The problem with writing a "fiction" class is that, although the typesetting is typically much simpler than (say) an algebra textbook, the variation in fiction typesetting is huge. For one thing, when we say "fiction," do we mean short-stories, novellas, novels, trilogies? Do we want named chapters or only numbered? Both? What kind of section separators do we want? And so forth. At that point, we might as well just have a
memoir class and trim it down.
As for publishers, I think one of the commenters already mentioned it: they just want very basic double-spaced type without much formatting. There are already classes to give this very basic appearance; e.g.,
manuscript (though that might get even more bare-bones than a publisher wants).
I hope this helps at least a little.