I just published a page (under the book companion pages) called “Freebies” where I am giving stuff away!
At this point, this page features texture images and spherical environments, which you can use for your SketchUp renderings. All of them are published under a Creative Commons Attribution 3.0 Unported License. Therefore, feel free to use any of them in your (personal and commercial) projects.
Although the environments are only LDR (not HDR) and most textures don’t tile, they can be very useful as the rendering above this text shows.
Keep checking the Freebies page (or my Flickr sets) often as I will be adding material regularly.