Update January 2022: a short presentation for an extension school course Math E 320 Spring 2022:
I had the opportunity this fall 2018 to learn more about Khipus (Quipu). Here is some write up (I learn best by writing) but be warned that I'm a complete layperson in the matter. The literature list gives some main references.

The Inka Khipu Enigma, [12 pages PDF]
From the Museum of Fine Arts, 2018:
Oliver Knill, December 2018