Some context can be provided by this earlier thread on slavery in China, with particular interest going to this chain with /u/Xuande88 on Chinese slavery, and my own contribution on Manchu slavery. But I can go into a bit more detail here on the specific matter of the Kangxi and Yongzheng manumissions.

The claim that the Kangxi Emperor engaged in a mass manumission of those enslaved to the Banners in 1685 is not one I had previously encountered, nor can I find it in the (admittedly somewhat limited) literature on Qing-era slavery, or in Mark Elliott's study of the Banners, The Manchu Way, which also includes a considerable section on slavery. A bit of cursory digging suggests that its most visible form is on Wikipedia, and can be traced to a tertiary work, the Encyclopaedia of Antislavery and Abolition from 2007, edited by Peter Hinks and John McKivigan, and specifically Nicole Hallett's entry for 'China and antislavery'. Unfortunately, Hallett seems to have made this factoid up: none of her three cited works actually mentions anything to do with a manumission in 1685. Indeed, just three years earlier, during the closing phases of the Three Feudatories Revolt, the Kangxi Emperor had approved proposals that the families of certain rebel generals were to be enslaved. While his private writings show some degree of compassionate concern for how enslaved people were treated, there is nothing to suggest that he fundamentally objected to slavery as an institution.

The actually-attested case of manumissions were the so-called 'Yongzheng Emancipations' which took place between 1722 and 1730 under the Kangxi emperor's successor, the Yongzheng Emperor. While conventionally presented as a form of abolitionist act, as the linked post by Xuande88 notes this was actually not a move against slavery, but arguably a reification of categories of freedom and unfreedom. The Yongzheng Emperor's concern was that tenants were being registered as serfs or slaves because of informal arrangements that had created stronger dependencies than what was supposedly codified. As Pamela Crossley's chapter notes, the 'slavery' against which the emperor was acting was not a codified form of servitude, but a de facto state of affairs caused by unchecked abuse of privilege by rural landlords.

The 'Yongzheng Emancipations' effectively consisted of the following:

• Affirming the free status of tenants;
• Affirming the free status of people in 'mean' or 'base' occupations, such as musicians, undertakers, couriers and fishermen (among others);
• Enrolling the above on the tax rolls;
• Enabling families that had been improperly classified as serfs or slaves to re-register as free;
• Affirming that it was illegal for landlords to physically or sexually abuse their tenants, or to prevent them from moving elsewhere; and
• Launching an inquest into landlord abuses.

It did not involve a fundamental objection to slavery itself. Moreover, many of the issues that had prompted these reforms remained, if on a smaller scale, until a firm statement was made in 1786, which declared that only those who lived in an employer's house and did household work could be defined as enslaved, whereas anyone who worked in the fields or lived outside the employer's house were by definition free. But, to stress this again, this was a case of increasingly strongly defining slavery on the understanding that it was, to the Qing state, normal under specific circumstances, and not an attempt to abolish the entire practice.

Indeed, this is where the booi aha come in. The reforms undertaken by the Yongzheng Emperor applied principally to Han Chinese landlords, and not to Manchu nobles, whose societal norms were rooted in both discourses and practices of slavery. Hong Taiji, the second Latter Jin khan and first Qing emperor, had limited the number of enslaved individuals to whom various nobles were entitled in order to affirm the hierarchies of the Banner system, but as the Qing established itself in the post-conquest era, slaveowning became a sign of Manchu status which affirmed Banner identity, although as Banner landholdings shrank, this was largely in the form of booi – enslaved household servants – rather than of aha – enslaved agricultural labourers. This I cover at greater length in the linked answer above.

I'm curious as to which book you've read, but as for which you could read, sadly the literature on Qing-era slavery is still relatively limited. Pamela Crossley's chapter in the Cambridge History of World Slavery is perhaps the best overview to also contextualise slavery in China alongside the Banner system as a form of military slavery; that latter aspect is covered in Mark Elliott's The Manchu Way. A systematic breakdown of forms of slavery under the Qing can be found in the form of a brief chapter by Angela Schottenhammer ('Slaves and Forms of Slavery in Late Imperial China (Seventeenth to Early Twentieth Century)'), in Gwyn Campbell's The Structure of Slavery in the Indian Ocean, Africa, and Asia, pp. 143–54.