Can you suggest me some unbiased, authentic books?

I'm not going to spend too much time on this, but simply point out that "unbiased" is at best an impossible goal (there is no such thing!), and at worst an actively terrible one (blind insistence on treating all sides neutrally). I've written on bias before here which lays this out in more detail.

As for your specific question, the KKK was a domestic terrorist organization whose goal was the terrorizing of black persons, in acts that ranged from harassment to murder, in order to enforce white supremacy in the American South. In later forms its targets expanded to include many other groups who didn't fit their nativist ideas of White, Anglo-Saxon Protestant identity. Any history book worth an ounce of good will be crystal clear about that. I realize that it can be hard to imagine people can be so awful as we want to believe the best in everyone, but unfortunately they really are. Since you mainly are looking for further reading, some academic works on the topic would include:

Ku-Klux: The birth of the Klan during Reconstruction by Elaine Frantz Parsons (Sadly, there are few recent academic works that focuses entirely on the First Klan. The Second gets most of the attention. This is the best you'll find).

Searching for Freedom after the Civil War: Klansman, Carpetbagger, Scalawag, and Freedman by G. Ward Hubbs

Black resistance to the Ku Klux Klan in the wake of the Civil War by Kwando Mbiassi Kinshasa

Behind the Mask of Chivalry: The Making of the Second Ku Klux Klan by Nancy MacLean

One Hundred Percent American: The Rebirth and Decline of the Ku Klux Klan in the 1920s by Thomas R. Pegram

And while not Academic, They Called Themselves the K.K.K. by Susan Campbell Bartoletti is intended for young adults, so may be a more basic, introductory text if you are looking for something simpler.