As an undergrad I wrote a book review for England in the Age of American Revolution by Sir Lewis Namier. The work was part of an ultimately unfinished series, with the narrative ending Part I in the 1760's if I remember correctly. It was a dense read, complex, and comprehensive. Has anyone ever finished this work? Do historians often or ever do this, or would this be taboo?
I can't speak to the general practice, but David Potter's The Impending Crisis: America Before the Civil War, 1848-1861 has a credit "Completed and Edited by Don E. Fehrenbacher". The copyright is 1976, but Potter died in '71. It's a proper academic history with all the usual footnotes that I've seen historians cite relatively recently. I can't speak to any controversy over the book's completion, or how much Fehrenbacher wrote himself, but if there was one it doesn't seem to have damaged the book's reputation.