Is there someone can tell me some papers or books about the basic material of Spectral flow? I want to know, what is spectral flow and how to use it to geometry.



An introduction paper or book to Spectral Flow
A good start could be to read this paper by Philips which was recommended to me, when I was looking for an overview:


[John Phillips, “Self-Adjoint Fredholm Operators And Spectral Flow”](https://www.cambridge.org/core/journals/canadian-mathematical-bulletin/article/selfadjoint-fredholm-operators-and-spectral-flow/CFA9F1362A48E77D9B67212CDCEFA902#).


