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Apple maintain a number of official WebObjects resources as part of the Apple Developer connection at http://developer.apple.com.
Legacy documentation is available as well. An obscure directory I came across is: http://mirror.apple.com/Mirrors/Apple_Software_Updates/MultiCountry/Enterprise/webobjects/guides/
There are also a number of third-party web sites where you can find out more about WebObjects and access articles and sample code.
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