Publication Strategy for book manuscript
With book proposals most authors submit proposals to several publishers at once and then negotiate with those that are interested in the book project.  There's generally no clause that says "You cannot have submitted this for publication elsewhere."  Although you can submit a completed manuscript along with your proposal, many publishers will just want to see an outline and perhaps a sample chapter or two.  They won't typically review the entire manuscript in the way that a research paper would be reviewed.