There are a number of stages to writing a book, each of which takes a great deal of time. You have to research it, you have to plot the whole thing out in your head, you have to write it, you have to sell it to an agent or a publisher and you then have to promote it to the potential readers. For most writers one, or at the most two, books a year are more than enough.
If you are ghostwriting, however, several of those stages are removed.
The research nearly always comes from one place – the author’s head or filing cabinet. The structure generally becomes clear during the research period, meaning you can go straight to the writing and seldom have problems with ‘writer’s block’, a condition which usually occurs when you are trying to force yourself to write before you are ready with the material. The ghosted book will also probably be an easier sell to agents and publishers, although not always, and you do not have to be involved in any of the promotional activity which happens months after the writing is finished, allowing you to get straight on with the next book without interruption.
As a result you can be actually writing any day that you are not meeting with the client, which gives you far more writing time than if you are originating all the material yourself from a variety of sources and then trying to sell it to the world.