In the field of history, books are based on the research a person conducts. It would be very difficult to write a decent book without having previously found research to discuss and analyze. I'm not sure the book would automatically lose all legitimacy, but it would certainly be suspect. It probably would depend on the author's credibility, background, topic, and the sources they used. If the sources are reputable and match the argument, it might not be as bad.

This isn't nearly the same thing, but I recently read Southern Cross by Christine Heyrman, and one thing that was pointed out to me was her primary sources. They were almost all memoirs or self-published diaries from when the individual had grown old. This doesn't take away the legitimacy, necessarily, but it requires more thought on the analysis of the evidence, as is could have been edited later in life.

Again, my example is not the same as your question. Heyrman won a lot of awards for the book. But an author has to be careful with the sources they use and how they use them.