Augustus Caesar Buell (1847-1904), employed by a major engineering firm, wrote several biographies of great Americans, following the success of a book about his experiences in the Civil War. Most material in these biographies that was not plagiarized was (as was discovered too late for many subsequent scholars) invented.
In 1876 he was briefly arrested following an accusation of libel.
In his Civil War memoir, "The Cannoneer", published in 1890, he vividly described his experiences at the Battle of Gettysburg, but that battle took place in July 1863, seven weeks before he enlisted, and two essays by Silas Felton reveal much of the hollow truth behind Buell's claims. He later claimed that a visit to St. Petersburg in Russia on behalf of Cramp's, a few years later, enabled him to find important material for his first biographical project, the life of John Paul Jones. The major demolition job on that work we owe to the far superior scholarship of Admiral Samuel Eliot Morison, who devoted an appendix to Buell's lies about Jones in his own Pulitzer-winning biography. The work of scholars like Morison, which should never have been necessary, reveals the full depth of Buell's fraud. Had he simply introduced false stories without citing any source document, there would have been less of a problem, but he went one step further, and invented entire letters and journals, from which he published extracts in the book.
Milton Hamilton, editor of the papers of colonial administrator Sir William Johnson, had to do the same sort of research as Morison to uncover the truth behind Buell's 1903 production. Two further biographies, of William Penn and Andrew Jackson, were published in the months following Buell's death, by which time suspicions of his work were already growing rapidly. His last posthumous work may be his most truthful, the memoirs of his boss, Charles H. Cramp. About the same time as this was published, Mrs Reginald de Koven produced the first detailed analysis of the fraudulent Buell technique, specifically his John Paul Jones biography.
Despite the suspicions, in 1905 a superb quotation from John Paul Jones had been copied from Buell's biography for "Reef Points", the handbook of the Brigade of Midshipmen. It was pretty much all Buell's work, based only loosely on genuine Jones material, as was his explanation of the context of the famous soundbite "I have not yet begun to fight"- but in both cases the lie achieved immortality because it was superior to the truth (and although Jones's reputation as "Father of the U.S. Navy" is largely based on Buell's fabrications, that does not mean it is entirely undeserved). The list of later biographies which innocently quoted Buell's invented primary source documents would make a substantial Wikipedia article by itself.
Buell's diary for 1867-68 still survives, in the Davidson Library, University of California, Santa Barbara