"The greatest honor that can be paid to the work of art, on its pedestal of ritual display, is to describe it with sensory completeness. We need a science of description…ceremonial revivification."
An abstract is a straightforward, appealing summary of your book. It can normally be one long paragraph, about 5-8 sentences. Even under special circumstances, however, it should not be longer than a page unless someone expressly asks for a long, chapter-by-chapter or section-by-section summation.
DVICE. Whether you are quite finished with your book or not, write an abstract. It will help you imagine the whole book if it does not yet exist, and it will help you refine the book if it does.
An abstract helps you to take control of your book's description, rather than handing that power over to a publicist or a copy editor.
An abstract becomes a good way of understanding the book; in that you will know what writing fits and what writing doesn't fit, the same way a good thesis statement helps a writer to understand what should be in an essay--and what shouldn't, even if it's good stuff.
An abstract often will become the text of other efforts--grant proposals and writer's colony applications, job applications, mentoring encounters, and so on. The abstract is just as likely to become the book's jacket copy, or author's note, or something. A good abstract will help the publicity department know how to talk about your book, and will help everyone in interview situations.
When you have pulled your manuscript together, do a "word count" analysis using your word processing software or a program such as Grammatik. This will tell you how often you have used each word. This inventory can be revealing, and useful. It may lead you to a title, or a better understanding of your content, or a last-minute editing epiphany.