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Here is BensAviSynthDocs, in its original form.
See more about the modifications for AviSynth 2.5 in the AviSynthTwoFiveSDK.
Also, SimpleSample has some very simple examples covering development of a filter that does virtually nothing through to one that draws a variable sized square, in the middle of the screen, in all ColorSpaces.
One thing not covered in SimpleSample, is how to ChangeFrameSize in a filter.
There are several different Colorspaces in AviSynth. See more information about ColorSpaces and WorkingWithImages.
Read more about the InternalFunctions in AviSynth.
You can also browse various topic on AssemblerOptimizing.
Once you've got the basics down on AVS development (Ben's text is quite good), the [SDK] for VirtualDub is also a good read. Good news is you won't have to worry about writing [function pointers] and [raw Win32]; meanwhile, Avery knows his stuff when it comes to video & CPU optimization techniques, so you best pay attention.
Some video related ebooks (PDF) can be downloaded freely from [Snell & Wilcox].
