The best-known text under his name is the Nahj al-Balagha, the Peak of Eloquence, a collection of sermons, letters and sayings compiled by al-Sharif al-Radi around the year 1000, roughly three and a half centuries after Ali's death. It is a work of extraordinary literary power and is central to Shia devotional and intellectual life. Its authenticity has been questioned since the medieval period, initially by Sunni critics and later in Western scholarship, on the grounds of the gap in transmission, the absence of chains for many pieces, and stylistic features that some argue belong to a later Arabic. Shia scholarship has defended it in detail. The honest description is that the compilation is tenth-century, that some material within it is attested earlier and independently, and that the collection as a whole cannot be traced to him.
Ali also stands at the head of nearly every Sufi initiatory chain, including in orders that are entirely Sunni. This is easy to misread as a Shia connection; it is not. Sufism traces esoteric knowledge through him because of his closeness to the Prophet, and the great majority of Sufi orders are Sunni.
At the far edge, some groups took veneration of him past what either main tradition accepts. The Alawites of Syria and the Alevis of Anatolia hold positions about him that Sunni and Twelver Shia authorities have both, at various times, rejected. These are distinct communities with distinct histories, and lumping them together is a common error.