There's enough documented cases where we KNOW this is the case to think this is likely true for a very large proportion of them. Dick Francis is a major case in point; his wife wrote most of "his" books.
With Dick Francis' books it seems to have been a public secret for a long time. It was a joint decision by the couple to market the books this way, where she was the one who insisted on not being credited, while he wanted to credit her.
"Most persuasive is an account of a conversation he had with Mary Francis following an interview with her husband in 1980, when he asked her directly if she had written the books.
That's an impossible question to answer," she said. "Yes, Dick would like me to have all the credit for them but believe me, Graham, it's much better for everyone, including the readers, to think that he writes them because they're taut, masculine books that might otherwise lose their credibility."
She begged Mr Lord not to mention this in his article, and he didn't, because he had no wish to damage their careers. But he did keep the notes from the conversation and now he thinks it is about time the truth was told, and she enjoyed some credit for her literary efforts.
"I don't think she wrote them entirely," said Mr Lord yesterday. "What I'm getting at is that Mary certainly has contributed a great deal more than anyone's ever previously suspected, and that Dick himself would like her to get very much more credit for the books."
https://www.independent.co.uk/arts-entertainment/books/news/dick-francis-thrillers-were-ghost-written-by-wife-743503.html