I understand where you are coming from with this question. Investigating and researching the topics around the Shi'a Sunni division can be mind boggling when there are claims and counter claims from both sides and the quality of evidence put forward by each side also gets challenged by the other.

Here is a suggestion to help you get started that is designed with your preference in mind of avoiding sectarian bias in your research.

Step 1

Start with the Nahj al-Balagha which is a collection of sermons and sayings by Imam 'Ali b. Abi Talib.  Although it was compiled by a Shi'a scholar named Sharif al-Radi but it attracted attention from many Sunni scholars who wrote commentaries on it. 

For instance see the list of commentators on this page, many of them are Sunni names - https://www.al-islam.org/articles/commentaries-nahjul-balaghah-syed-waheed-akhtar

The reason for the wider Sunni interest was that the sermons included by Sharif al-Radi could be traced to other sources acceptable to both the Shi'a and Sunni communities.  And, perhaps more importantly, a large part of the value of the book was in the eloquence of words and thoughts expressed by 'Ali who was a master at it, being the close disciple and confidante of the Prophet Muhammad (s). In fact, for many, the eloquence of the words also testifies to its authentic origins.

You can find the Nahj al-Balagha here - https://www.al-islam.org/nahjul-balagha-part-1-sermons

When you go through this book, and I do suggest you go cover to cover, you will gain an insight into what 'Ali was saying to the people of Kufa during his time as the fourth caliph. You will get a sense of his views on the disputes, the civil wars, the various famous Companions and mothers of the believers who often get mentioned in Shi'a Sunni debates.

Step 2

Once you are through that, it is time to understand the entire history of the debate on the succession to the Prophet Muhammad (s). Contrary to what many people realise, a good understanding of the period of the civil wars during Imam 'Ali's rule is essential to understanding and evaluating the reports on the earlier period of Islamic history.

In order to avoid any intentional or unintentional sectarian bias by a Shi'a or Sunni author, I suggest you instead go through a book called Succession to Muhammad - A Study of the early Caliphate by a famous non-Muslim academic called Wilferd Madelung.

This book does a good job of looking at the often contradicting reports on early Islamic history and analyses the likelihood of bias of individual narrators and, therefore, reports. Although somewhat heavy reading, it will help you get a pretty good idea of what really happened back then in early Islam.

Good luck with your research.