In addition to my answer on Islam.SE, if you think non-Muslim sources may be less "biased" (I agree many Muslim sources can be biased on this, even though I believe the Shia view is accurate), you may want to read the book by Wilfred Madelung, a German-British scholar of early Islamic history who has confirmed the idea of superiority of Ali over the caliphs and his disputes with them.
Note that even Western scholars are divided on this to some part. Madelung's work though is among the most recent works which builds up on previous scholarship but also criticizes them, giving qualified support to the Shia belief. But he diverges from the Shia view that the Prophet Muhammad had actually appointed Ali as his caliph but he argues that hereditary succession was always a given in Abrahamic traditions as well as Quranic account of succession to past prophets.
PS: In reference to comments, the idea that the first three caliphs were "elected" is also contested. If the electoral excludes some of the most prominent companions of the Prophet and involves coercion, it can't be called an "election" and this was the case for Saqifa the first "election" which was more like a factional coup d'état. The second caliph was also appointed not elected. You may want to post a separate question on this but you can also consult Madelung's comprehensive account of post-prophetic politics.