I believe that every artist should have a special sketchbook/journal that travels with them and becomes their trusty companion. I have found my journal, and already it has stayed with me and I've sketched in it quite a bit. It helps me with my drawing skills, and is a great source for me to look back at the tehniques I've learned.
Here are some of the sketches I've done in my book so far. (I left in any guidelines, since they're just sketches)