Publishing a photography book is extremely difficult. The very few opportunities that exist result in huge competition on every publisher's desk. The key is to submit a well-considered, illustrated proposal to an appropriate publisher at the right time.
The simplest way to find a suitable publisher is to spend a morning in a large book shop. Note the publishers of recent books that most closely resemble your own proposal. Bear in mind that many large publishing companies own smaller ones - it is all too easy to send three copies to the same publisher! Simultaneous submissions to several publishers are a reality these days, but publishers understandably do not like them. They may spend six months considering a proposal only to find that the author has just signed a contract with another company. However, the chances of two acceptances are negligible.
The development a good book proposal requires a lot of work - usually spread over a period of months. A publisher will expect to see every conceivable aspect of a subject addressed so as to maximize the sales of the book. However, if the scope becomes too wide there will be room for only cursory consideration of subject matter. The balance must be right.
A basic photographic book proposal consists of a contents list, a detailed synopsis packed with carefully-structured ideas (maybe 1,500 or 2,000 words), a sample chapter complete in every respect, and appropriate captioned illustration for the sample chapter. This may sound simple but should, when done properly, occupy an author for a considerable time.
The style of the written content is very important, and grammar, spelling and punctuation should show an appropriate grasp of the language. Small mistakes and typos do not matter as these will be corrected by a project editor, but muddled thinking, poor structure and repeated errors soon undermine the credibility of an author.
Images should be submitted in a manner similar to that described for magazine publishing. However the the aesthetic and technical quality of the images should be even higher.
Interfacing with the book-publishing world can be extremely frustrating. Most proposals are rejected quickly, but some are retained for more careful scrutiny. After three to six months it is probably reasonable to request some feedback, but publishers can take years to make decisions. When finally successful, don't expect to make a fortune. Most authors receive virtually nothing for their years of work.