The VBScript reference that comes along with the QTP help files is not a bad place to start at all. Especially since it gives you a side-by-side look at VB scripting and the object model. For general scripting style guidelines you could pick up a simple C or VB book.
If you're beginning your adventures in QTP expert view, start by simple record-and-play and enhance these scripts using the VB scripting help file as a guideline.
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