Most people write because they enjoy it, but never stop to think if their work is marketable. Whether publishing yourself or seeking a publisher through submissions, your book must be in a genre or on a topic that will sell!
In his book "Publishing For Profit", Thomas Woll recommends defining your editorial niche.
"Ask yourself the following questions:
What makes your book unique within the market?
Do you have particular expertise within this market?
Where in the store is this category placed?
What are the attributes of competitive books?
What is the average number of copies sold within this category?
Are sales in the category expanding or contracting?"
Too often writers select a topic or genre that has a saturated market or virtually no sales. ( Such as poetry and autobiographies of non-famous people - they do not sell.)
So do your research before you pour your heart & soul into a book!