If an editor has agreed to review a proposal on its own, you may get a response in a month or so, as a short proposal does not receive a lot of scrutiny from reviewers. If you have submitted a complete manuscript, six months is not unusual. It takes a long time for a peer-reviewer to find the space to give a full book manuscript the attention it deserves. If you have waited that long, however, you ought to get some decent feedback – several pages from each of 1-3 referees.
Unlike journal submissions, a book manuscript won’t typically be reviewed double-blind; you won’t know the reviewers’ identities, but they will, inevitably, know yours. Like a journal submission, the editor will then either decline your manuscript, accept it outright (while encouraging you to address the referees’ comments in developing your final version), or give you the equivalent of ‘revise and resubmit’ – encourage you to go away and re- think the manuscript in light of the referees’ comments. This latter verdict is not as common as in journal submissions – it is asking a lot to get an author to rewrite a whole book.
As with a journal submission, once accepted your manuscript will go through copy-editing and proofreading. A good publisher will proofread your book themselves and expect you to do it too. As with journal submissions, you have to confine your major changes to the copy-editing stage. And there’s an additional stage as well – indexing. Most first authors do their own index, at the same time as they are proofreading. There are good software packages that make this easy. Some journal articles have illustrations, and it’s the author’s job to source (and pay for permissions for) illustrations. All the publisher does is provide the technical specifications (nowadays, what kind of image file is required). This task is more onerous for books, which often have a lot of illustrations.
Only the wealthier publishers will even offer to help with sourcing and paying for illustrations. There are charitable trusts that you can apply to for subsidy – ask your publisher for advice. Most books will require at least a cover illustration – again, probably your responsibility. You’ll also be asked to supply jacket copy – including the ‘blurb’ describing and touting the book – although normally publishers themselves secure endorsement blurbs from senior scholars (often by asking those who refereed your manuscript or even excerpting the report).