For the study of integral equations to be used in numerical methods, you can use the codes in Fortran 90 book Numerical Recipes:
Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; and Vetterling, W. T. "Integral Equations and Inverse Theory." Ch. 18 in Numerical Recipes in FORTRAN: The Art of Scientific Computing, 2nd ed. Cambridge, England: Cambridge University Press, pp. 779-817, 1992.
You can see the link:
http://www.nrbook.com/a/bookf90pdf.html
To study theory of integral equations, you can also use:
Corduneanu, C. (1991). Integral equations and applications (Vol. 148). Cambridge: Cambridge University Press.
Gripenberg, G., Londen, S. O., & Staffans, O. (1990). Volterra integral and functional equations (Vol. 34). Cambridge University Press.
Burton, T. A. (2005). Volterra integral and differential equations (Vol. 202). Elsevier.