The book you're writing might be a good book. But could it be better?
Writing fiction is different to editing fiction. Each activity requires a separate frame of mind. Writing is creative thinking, editing is critical thinking. When writing, all you worry about is the narrative flow, not the consistency of point of view or the number of adverbs in your manuscript.
Self-editing Fiction That Sells will teach you the necessary frame of mind to sit yourself in 'opposition' to your manuscript, as an editor would do, and improve it through structured editing and rewriting.