Book publishing remains fairly traditional, not as affected by the digital revolution as journal publishing. As with journals, however, there are a range of book publishers that you can probably array in a quality sequence depending on your own experience of your own field. Generally, though, they break down into three types: i) university presses; ii) big commercial presses; iii) boutique commercial presses.
The university presses tend to carry the most prestige – because they do the most thorough peer-review, and thus the most thorough quality control. Their mission is to publish academic books (and so are not under such pressure to rack up big sales or require ‘crossover’ appeal). The big commercial presses need higher sales; they’ll only be interested in an academic book if it can be dressed up with ‘crossover’ appeal, either stylistically or substantively (sex, violence, war, or some other kind of rich human interest).
Boutique commercial presses have burgeoned in recent years, as more academics want to publish more books than the university presses can accommodate. They rely on the fact that they can make a small profit with modern technology by selling only 150-350 copies of an academic monograph at a high price to a small number of libraries. All of these presses still have editors who go around universities and conferences prospecting for authors. You can often meet them at book displays at conferences.
Mostly, however, you will have to approach an editor yourself. The best way to do this is to ask a senior colleague or a mentor for an introduction, or at least for permission to use their name when you email an editor with a query. As with journals, the best way to choose a press is to identify which press publishes books in your field that you admire and wish to emulate.
Publishers differ in their practices, but many presses will accept an initial submission in the form of a proposal. This could be a 5-20 page summary of the work, which tells the editor what the book is about, why it is new and special, what kind of research it’s based on, and something about the structure of the book (a chapter outline with abstracts of each chapter). It’s better to accompany the proposal with a sample chapter, which you’re already pretty happy with.
The university presses will almost certainly require you to go through peer review on the basis of a complete manuscript. They may issue you a contract but it will include a clause reserving the right to accept or reject the final manuscript on the basis of peer review.