Everybody loves a moving target. No, wait... let me try that again. Nobody loves a moving target. Unfortunately, when it comes to word count, it's definitely a moving target. When you as a search engine, "How long should my book be?" you'll get umpteen answers — and they'll all be different.
This is in part related to genre, in part related to the many ways publishing is changing, and in part because word count recommendations are only ever guidelines; never rules.
The best rule is to write and edit a tight, well-told story. Then, however long it is, it's right. But, since everyone has to put together a list, here's one from our experience:
Historical: 80,000 - 100,000 words (these readers like longer books.)
Romance: 45,000 - 80,000 (these readers are the type who jump straight into their next book as soon as they're done.)
Mystery / Crime: 70,000 - 80,000 (keep it moving for these readers.)
Sci-fi/Fantasy: 80,000 - 110,000 (Nerds like me don't mind a slow burn.)
Literary / Contemporary: 60,000 - 125,000 (These readers care about style as well as story, but don't worry as much about page count.)
Young Adult: 50,000 - 80,000 (And yes, we know all the exceptions to these rules.)
Middle grade: 30,000 - 50,000 (Trends short and tight. Remember, these are student readers.)
Picture books: 300 - 700 words.
Christian Living: 50,000 - 75,000 (And we're seeing this trend downward. Lots of short, to-the-point books hitting the market lately.)
Bible Study: 25,000 - 35,000 (These tend to be about guiding the reader to do work, rather than doing the work for them.)
Theology: 80,000 - 120,000 (Tends to be an academic reader. There is a surge in short theological books lately, though.)
Memoir: 60,000 - 80,000 (These can't just be "look what happened to me," but have to take the reader on a journey of thought and understanding.)
Pretty sure we're wrong? Tell us in the comments! There's no way to be exhaustive when it comes to word counts — and no one knows it all!