Transform Calculus

Fast Fourier Transform
Article in MathWorld.
http://mathworld.wolfram.com/FastFourierTransform.html

Fast Fourier Transforms
Survey and history of FFT algorithms from Wikipedia.
http://en.wikipedia.org/wiki/Fast_Fourier_transform

An Introduction to Fourier Theory
A mathematically inclined introduction with illustrations and explanations of related theorems, its properties and numerical methods for calculating transforms.
http://gershwin.ens.fr/vdaniel/Doc-Locale/Cours-Mirrored/Maths-Stuff/fourier/

Introduction to the Fourier Transform
Notes on applications to Image Processing.
http://www.cs.unm.edu/~brayer/vision/fourier.html

Mathematics of the Discrete Fourier Transform (DFT)
Notes by Julius O. Smith III, Stanford.
http://ccrma-www.stanford.edu/~jos/r320/