Consider your audience. An often-repeated reminder: your dissertation or thesis cannot be repackaged as your first book without significant revision. Many students find themselves in a position where they are writing primarily for their director and committee, each of whom plays a critical role in the student’s success. If your committee does not see your project making a meaningful contribution to the field, you may get sucked into a spiral of revision that keeps you from completion.
Once you have succeeded and graduated, your audience changes. Do you have a publisher in mind? A press that you would most like to put out that first book for you? Take a close look at what that press publishes. Will your manuscript be a good fit? Is there a particular editor to whom you would submit the manuscript? What books are in that editor’s repertoire? The degree to which you would write toward a particular audience/market changes from one discipline to another, but it can be helpful to bear in mind that an editor will need to know your book is marketable before offering you a contract.