I always give new colleagues "Adobe RoboHelp 7 HTML: The Essentials" by K.A. Siegel. Easy to use with examples and test projects. There's a RH8 version: "Adobe RoboHelp 8 HTML: The Essentials". You can get this via amazon.com.
You can also take a look at Rick Stones' Gooberguides: http://www.gooberguides.com/. I haven't used those, but if those books are as good as Rick's advice on the forums, they'll be worth every penny.
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