This is where The First Five Pages: A Writer’s Guide to Staying Out of the Rejection Pile by Noak Lukeman comes into its own. It tells you what you should not do as well as what you should. Sometimes knowing what you should not do is actually far more useful. What Lukeman clarifies that agents, are looking to reject a manuscript. That thought will horrify you perhaps, but he provides the landmarks you need to follow to ensure your book is not thrown into the rejection pile.
Although I recommend this book, I have a proviso; it is not suited to all writers. It is geared towards genre/commercial fiction in the main and not literary fiction.