My 10 year old niece enjoys writing fiction and writes very well for her age. I would love to encourage her love of writing.
Can anyone recommend a good fiction writing book that is accessible to a 10-year old?
First of all, it's great of you to take such an interest in your niece's passion for writing! I'm sure it's much appreciated :)
To the main point, I'm not finding much in the way of books on writing for children. And I think that's for good cause: nobody's trying to get children to write fiction particularly well. Kids at that age aren't really in the running for writing good fiction, and I don't think anybody's trying to push them in - just encourage them to keep enjoying it. For that, you don't want a how-to book, any more than you'd want to get a ten-year a personal trainer just because he enjoys playing basketball.
There are lots of other ways to encourage a kid's enthusiasm for writing, which might be more effective. Obviously, this will depend muchly on the kid herself, and your relationship, so YMMV.
I did find one book that looks right up your alley: Kids Write!, by Rebecca Olien. It looks really aimed at kids (and not at kids' teachers); it's also got good Amazon reviews. Probably worth checking out! Following links on Amazon leads to other likely candidates - Writing Magic and Spilling Ink: A Young Writer's Handbook both caught my eye. I can't personally attest for the quality of any of these, but browse for yourself and see what might suit your niece's fancy :)
I'd also recommend trying The Breakout Novelist. It uses many examples from current novels, is easy to read, and has exercises to try at the end of each section. It covers both the core concepts of writing a fiction novel and then expands on them in more detail afterwards.
It's actually a combination of three writing books also written by the same guy, all of which I enjoyed a lot.