If I am going to be honest, the best study on the Boxers is absolutely The Origins of the Boxer Uprising by Joseph Esherick. You will not find a better book on the topic. It also is “sympathetic” to the Boxers (not even banned in the PRC) because it is very thorough in providing historical context.

The fact of the matter is that the Europeans and their religious converts in Shandong province (where the events originated) were mistreating the people who lived there. What people miss is that this was not about anti-imperialism or xenophobia. It was simpler in a way - the Boxers were not stupid; they rightly understood that religion and politics were tied together in how the Westerners treated them. Christianity became associated with oppression because the Westerners used Christianity as an excuse to treat Chinese people (specifically the people of Shandong - this was a very personal event, not a patriotic one) poorly. The goal was to take land from people in Shandong and the Western powers were actively looking for excuses to do so; the Christian missionaries in fact treated people poorly in hopes of starting a major incident.

Another great book on the Uprising is Paul Cohen’s History in Three Keys: The Boxers as Event, Experience, and Myth. It makes an argument about how historical scholarship should be conducted, then uses the Boxers as a case study. To me the most interesting part is the third “myth” section because it shows how the Boxers have been used by both Westerners and Chinese in various ways to support certain ideologies and viewpoints. Very fascinating.

These two books are the Boxer Uprising books, and you need to read them if you want the most thoroughly researched studies. Studies sympathetic to the West are simply not very accurate, frankly. I wouldn’t even call Cohen’s book sympathetic to the Boxers since it isn’t about the origins of the conflict. Esherick, though, is not sympathetic to the Boxers because of ideology; it’s just that the full picture really doesn’t end up making Westerners look good. Hardly surprising considering the position of Westerners in the rest of Asia at the time.

u/KippyPowers is correct that most contemporary historiography takes a sympathetic view of the Boxers. The Esherick and Cohen books are fantastic and I would highly recommend reading them even if you disagree.

If you're looking for ideologically slanted pro-missionary books, you're in luck! The two that I remember reading as part of my dissertation were Robert Coventry Forsyth's The China Martyrs of 1900 (1904) and Isaac Taylor Headland Chinese Heroes in the Boxer Uprising (1902). These are books that unequivocally promote a pro-missionary, pro-Christian narrative of the Boxer Rebellion. They're interesting historical documents, but better records for how Westerners depicted the uprising than an accurate accounting of it. If you're looking for more information, really any book discussing the China Martyrs of 1900 will depict the uprising in the way you are looking for.