These are good reference books, and you can have a quick preview read on the internet as well
David Busch's Nikon D3100 Guide To Digital SLR Photography
David Busch's Compact Field Guide for The Nikon D3100
Nikon D3100 From Snapshots to Great Shots
I have the dummies guide and found it too simple and have hardly picked it up in months so is almost new, maybe for somebody who had never picked a camera up in their life. But assumes you really are a total beginner and haven't the slightest idea (or inclination) to sit and just play with the settings and dials to see what does what