title_body: Genres/ Subjects/ Categories in epub's metadata
upvoted_answer: This field comes from the Dublin Core (DC) metadata standard, which is useful for interoperability between software and databases. I don't think there has emerged an accepted way to use DC metadata for ebook reading systems.   
  
  One idea might be to go with BISAC categories (as listed here). My memory could be mistaken, but I don't think the reading systems process these values in any meaningful way, (wouldn't that be nice if they did!) BTW, I seem to recall that Google Play required that you use BISAC codes when selling through them. For the DC metadata, don't use the alphanumeric value of the BISAC,  just use a simple phrase based on BISAC values. 
  
  Here's a book marketing guy explaining BISAC vs. Amazon's categories. Here's  more by this same book marketing guy.