Now that you have found an editor and publisher willing to "look at" your work, you need to produce it.
As has already been mentioned, no one "writes books." However, volumes are put together.
What, you may ask, does this phrase refer to?
Take all of the material you have composed in your young academic career: your dissertation, articles, book reviews, coursework, your comprehensive exam essays, your undergraduate thesis, idle notes on bar napkins -- and put it all in a 10 x 13 clasp envelope preprinted with your academic institution's address.
Insert a cover letter explaining that you have pitched your monograph to the editor, and have included it for consideration. Explain that the editor invited you to submit this manuscript.
Add to your c.v. an item stating that you have a work under review.