Books

This is a fantastic book that shows how to get your models from the conceptual stage all the way to drawings. Matt Donley also includes many extra resources like textures and tutorial videos.

Bonnie Roskes has many books available. Her Sketchup talents range from 3D printing to education. It's definitely worth checking out her site, especially if you're an educator.

Sketchup Sites

This is the official site of Sketchup. Here you can download the free version, take a demo of the Pro version, or buy Pro. 

This is Sketchup's official YouTube channel. This is a great place for you to get started with their free tutorial videos.

Sketchucation is a great forum and resource for the power user, or someone wanting to become a power users. The people that hang out here have an amazing amount of Sketchup experience.