Let's see. Is fan modifying fiction or is it a compound noun? Other genres are described as science fiction or romance fiction. But mysteries aren't generally called mystery fiction. OTOH, it's romance novel, science fiction novel, mystery novel. OTOH, fans can write their stories in any genre.
I come down on the side of one word, but that's just me. vamysteryfan
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I come down on the side of one word, but that's just me. vamysteryfan