Publishing houses, especially mid- and small-size publishers, often don’t have the personnel to handle every single facet of producing and marketing a book. They may out-source some phases of the process, such as photography, editing, artwork, research–even writing, to a BOOK PACKAGER.
Sometimes this happens when a publisher has an idea for a book and hires the packager to develop it from the ground up. Sometimes, the packager has the idea and pitches it to the publisher. Books developed this way may be either fiction or nonfiction.
A good reason for using a book packager is that a nonfiction idea (for a textbook, for instance) may require a lot of research, fact-checking, access to resources, etc. A packager can recruit people for every phase, up to and including the actual printing of the book (a particular process for specialty books like coffee table art books). Each of the people commissioned for this work is paid a flat fee, as opposed to a royalty. They may or may not be named in the credits for the book, depending on their contracts.
Most fiction handled by packagers is series fiction, in which a world and characters have been well-established and authors are needed to turn out timely sequels. Publishing houses may develop a synopsis or outline of each book needed and the packager finds the appropriate talent to execute the writing. Examples of this type of series include Nancy Drew, The 39 Clues, Sweet Valley High, and Goosebumps. Authors may be named (The 39 Clues) or not (Nancy Drew), and usually earn only a flat fee.