A nice beginning are some of the articles is the link below.
In particular the "A Software Defined Radio for the Masses" articles by Gerald Youngblood found towards the bottom of the page. http://www.arrl.org/tis/info/sdr.html there are many other useful articles at that location.
Articles how Quadrature signals work.
Unfortunately several links in this section went dead so I will need to look for additional links on how Quadrature signals work.
A free book on how DSP works, an excellent book.
Math Simulation software SciLab, software for analyzing data and plotting the results, very capable, can work with .WAV files as data. http://www.scilab.org/
Modeler for SciLab
SciLab Tricks and tips.
Octave another math modeler
Talk and models by Phil Hartman from talk at Dayton, it uses Excel to model SDR software, a good source of equations for the programs mentioned above. http://www.hamsdr.com/dnld.aspx