Imaginism Studios is a group of highly talented artists that specialize in pre-production visualizations and children's book illustrations. They're also running a really cool artist school called
Schoolism. These artists have worked for the likes of Disney, Sony, and Warner. Bobbi Chu has written an awesome book on how to become an artist called "The Perfect Bait" (available for download from the iTunes app store). I love pictures that tell a story, and these guys are pros!
They also have an app that allows you to download their printed sketchbooks; I've picked up a few of them. I love seeing the raw sketches, and learning about what the artist was doing while they were sketching.
You know what else is awesome? They're based in CANADA! Toronto, actually. But still cool.