Some of the classic texts:

Warren, Bertram Eugene. X-ray Diffraction. Courier Corporation, 1969.

Guinier, André. X-ray diffraction in crystals, imperfect crystals, and amorphous bodies. Courier Corporation, 1994.

Squires, Gordon Leslie. Introduction to the theory of thermal neutron scattering. Cambridge university press, 2012.

You may also find useful:

Keen, D. A., A Comparison of Various Commonly Used Correlation Functions for Describing Total Scattering. J. Appl. Cryst. 2001, 34, 172-177.

Fischer, H. E.; Barnes, A. C.; Salmon, P. S., Neutron and X-Ray Diffraction Studies of Liquids and Glasses. Rep. Prog. Phys. 2006, 69, 233-299.

@ Oliver L. G. Alderman Thank you very much for the reply.

My organic compound is  a solid.  Can we get the information that the compound is a solid from RDF? What is sigma? (it is repeated seen everywhere in RDF).  In most of the graphs it is r/sigma and NOT r alone.