Some Good Tutorial and Implementation will helpful to understand Fast Fourier Transform .
You can refer to this link. It contains different variations of -FFT along with the code snippets.
This link also contains the description of FFT in a very helpful manner.
you can try this video tutorial-: https://www.youtube.com/watch?v=qrGBpexbT-c
you can also learn from e-maxx http://e-maxx.ru/algo/fft_multiply
(translate it with google translate)
Thomas H. Cormen, Charles E. Leiserson, Ronald L. Rivest, and Clifford Stein “Introduction to Algorithms, 3rd Edition”, Chapter 30 - Polynomials and the FFT
Jörg Arndt “Matters Computational”