Occasionally a dissertation, as is, can be accepted for publication as a book by a regular press. More often, though, one of two things will happen. Often the publication needs to be subsidized, since sales are not expected to meet the full cost. Students should inquire if their institutions have funds for such purposes. The other route for publication of a dissertation is to make revisions, changing the work from typical dissertation format into book format.
Dissertations tend to be . . .
When attempting to publish a book, you can send the entire typescript to one publisher at a time, or you can send a book proposal to a number of publishers. Also send the table of contents, if the description does not break the work down by chapters, and a number of pages as a writing sample. There are two advantages of sending a proposal. First, of course, you can send the proposal to as many different presses as you like. Second, you can be pursuing a publisher while you finish the book.