Remember, first and foremost, opinions vary hugely in this arena between different sources. However, most recognised professional sources are approximately the same. Here's a guide from 'The Write Life'.
The following are average word-count ranges by genre.
For a great discussion on the fundamentals of fiction, and how to write a great story, check out this podcast interview.
All of these are average book word count ranges and should not be taken as the definitive word count you must reach in your book. We all know of outliers within each genre that have been published well under, or well over, these word counts.
Use these numbers as a baseline for your writing goals.
Know what readers expect in terms of your genre’s word count (even if the reader isn’t aware of their expectations when it comes to how long a book is).