Book publishers are the traditional gateway through which an author publishes a book. A book publisher was once deemed indispensable if you wanted to get your book in front of readers where they might buy it. But actually, it wasn't always that way, and it isn't that way now.
Until the 19th century, authors typically published books themselves. They would have books printed by a commercial printer, and bound by a bookbinder. Then the author would market the book, going from shop to shop to get it on shelves; advertising the book in newspapers and magazines; and writing handwritten mail offers to friends or strangers. It was hard work, but the author got to keep a large share of the profits, and profits were substantial because books were relatively rare. That all changed when book publishers came along.
A book publishing company takes all the pain out of publishing a book -- and most of the profit too. The book publishing industry promises authors that they will be able to "just write," and the publisher will handle all the mundane work of typesetting, printing, marketing and distributing the book. In exchange, the publisher keeps most of the revenue and pays the author a royalty of a few percentage points on the profits. Few authors ever see significant royalty income, because few books turn a profit after all the expenses that publishers incur. And the "you can just write" part is bogus, too.
Write a book for a book publisher and you will be asked to help market the book. You'll have to write to all your acquaintances touting your book, get as many as you can to submit favorable comments for the book jacket, travel around the country promoting your book, and generally doing all those things you thought you were paying the book publisher to do.
The book publishing industry does not help authors become successful. Today, the average book published in the United States sells 250 copies annually and less than 3,000 copies over its lifetime. You'll have to write a dozen books per year to earn a decent living that way. It's unlikely you'll be able to write that many and have them be good.
"Does this include Children's Book publishers?" people ask us. Yes: just about all children's book publishers are owned by large publishing houses, and they do business in the same way.
Thus, authors are beginning to realize they don't need book publishers; or, at least, that book publishers don't do enough to earn the lion's share of revenue from a book. Authors are increasingly returning to self-publishing. They are becoming fully rounded business people instead of just writing machines.
Publishing your book is not a do-it-yourself project. You need partners who can edit, typeset, illustrate, print, and bind your book. But you can find and deal with these partners on your own without paying a middleman book publisher. You can also eliminate the physical production and distribution of books by publishing books in electronic form, using book publishing software.