I am a ghostwriter, so I can tell you from my perspective. You're welcome to ask me anything
It all depends how it's done. If the pastor (or client) has the ideas, the content (sermons, presentations, outlines etc.) it makes perfect sense to hire a professional writer who knows how to put books together, how to organize the content, and keep their original voice while doing so. It's all pretty above board this way. They are the 'author' - the 'authority - and writers and editors come around them to help them refine their content.
It's not above-board when the author simply has one idea and hires someone to do all the research, all the theology, etc.
Ghostwriting is quite an old profession. They are really scribes. In the New Testament times, scribes were used to help people write letters (legal, or otherwise) and craft content for people who paid for it. A ghostwriter also functions as what we call a 'developmental editor'.
You might recall that Paul and Peter make mention of scribes who helped them craft their letters? (Tertius, Silas.) Those letters were not a 'one-man' endeavor but were a team effort.
Almost every creative industry makes use of background professionals to get something done. For example, most popular music artists make use of songwriters and producers. These craft and make more changes to songs than most people realize. The music is marketed under the artist's name and most people don't know about the songwriters, the producers, the engineers, the choreographers, etc. that go into making the final product.
When it comes to visual art, things are a little different in certain niches (yes) as mentioned with Picasso. However, there are still curators etc. who get involved. And bear in mind, a lot of artists in the past had benefactors who came up with a concept and hired artists to do it.
The book publishing industry is much the same and has been since its inception. There are writers, editors, researchers, developmental editors, market editors, agents and other publishing professionals who all contribute to one book. An 'author' is called as such because they are the 'authority' - but not necessarily the writer or the editor, or even the principle researcher (although they should be involved with all these things). This isn't a case of fooling the consumer - it's more a case of the consumer just not knowing how it works. But why would they? Do we all know the background work and all the people who help our sports stars become the people they are? No. Most people don't even care!
The only reason why you might feel cheated is simply because you didn't know, not because it's unethical or someone is deliberately trying to fool you.
