Publishing a book with other's photographs. How does the copyright/royalties work?

If you contract people to take pictures for such a collective work, in writing, you can call them "works for hire" and you become the "author" of those pictures. Otherwise, you would need a license from the author or whoever owns the copyright for the picture, or a clearance that the picture is not copyrighted. Each one can be separately negotiated, but more likely you or the publisher would go to a clearinghouse, pay a calculated fee, agree to a calculated royalty, and leave it to the clearinghouse to find and pay the copyright owners their share.