Saying "I wrote in my book that..." is basically the same value, in the absence of further information. We don't know how qualified the author is to make such claims, what the motive behind writing the book was, what sources they used, and so on. Even if you read the book, you might not satisfactorily find that information, or sources may themselves be misrepresented. Anyone can write and publish a book these days, making any claim, misinterpreting data, wilfully or otherwise.
If the person you replied to wrote their anecdote about their nephew in a book, would you find it credible then?
