The Mishnah Berurah (MB).
I went to a shiur by Rabbi Hershel Schachter who discussed the disputed authorship of the MB. He had heard evidence from a friend/colleague (I can't remember who) that the MB was compiled by close relatives of Rabbi Yisrael Meir Kagan (aka the 'Chofetz Chaim') and that Rabbi Kagan himself was merely the editor.
When the Chabad library was famously released by court order in 1986 from Russia ("Hey Tevess"), many books and manuscripts were returned. Rav Schachter recounts that he knew that an original manuscript of the MB was among the collection and asked a colleague to seek it out, to check for evidence regarding it's authorship. The colleague confirmed that different sections of the MB manuscript were written with different handwriting (he didn't specify how many) styles and inks, and that the 'easily recognizable' style of Rabbi Kagan was intermittently found on some margins on some of the pages within the manuscript.
Rav Schachter then noted some of the 'famous' discussions around the internal contradictions within the MB. Admittedly, he said, we cannot know whether the post-hoc resolutions of these discrepancies are an accurate representation of what Rabbi Kagan had intended, or whether they are a result of subtle differences in the opinions of the multiple authors; that were hard for the editor (Rabbi Kagan) to fully resolve. Teiku.
I can't vouch for the veracity of the anecdotal claims of the story, but it is, more or less, how Rav Schachter recounted the story.
Also I am not specifically interested in books whose authorship is simply unknown or a matter of speculation. This question is specifically about a dispute.What exactly separates unknown and speculated form dispute. Generally if something is subject to speculation, different people will suggest different possibilities. Are you only referring to those works of questionable authorship have been hotly debated to high stakes involved? – mevaqesh Oct 13 '16 at 15:37