Philip Corso. About Rosswell events. Wikipedia.

The Day After Roswell is an American book about extraterrestrial spacecraft and the Roswell UFO incident. It was written by United States Army Colonel Philip J. Corso, with help from William J. Birnes, and was published as a tell-all memoir by Pocket Books in 1997, a year before Corso's death. The book claims that an extraterrestrial spacecraft crashed near Roswell, New Mexico, in 1947 and was recovered by the United States government who then sought to cover up all evidence of extraterrestrials.

 The book appeared on The New York Times Best Sellers List for several weeks,but received many favorable & unfavorable reviews. Publishers Weekly advised Corso's book is only for the few special libraries that have made documenting the unconventional a collecting priority.
In 2001, The Guardian included the book in its list of "Top Ten literary hoaxes".

There is no conclusive proof as of 2020 whether the book is non fiction or fiction.