Even before you've finished the book, you should be following what is happening in the publishing world,and in your genre specifically. Accomplish this through membership in writers groups and attending conferences where you can meet and network with other writers. Don't write in a vacuum. Do your homework even before the book is finished.
I'm a serious writer, meaning I have a regular daily writing habit, and I'm interested in sharing my work through publication. My favorite literary form is the novel. I write to entertain myself and my readers.