John Cleese videos, esp. "Creativity in Management" on YouTube
"The Creative Habit"
"Creativity: Flow and the Psychology of Discovery and Invention"
If you really take it to heart, though, the Cleese video is all you need. Creativity in my mind falls into the category of "simple but not easy": you only have to do a few things, but they require hard work and thought.
1. Create time and space. Easier said than done if you have a busy life and people who expect things from you. You need to make a quiet place you go where you are not interrupted for a specific period of time. There you think, imagine, focus on your goal and work.
2. Mental clarity / confidence to play and do something not "serious." Creativity requires a playful mindset and the confidence to overcome your self-doubts and fears. Again easier said than done.
3. You have to work. Creativity is a cycle of work, examine the thing you made, and come up with more ideas / next steps. If all you do is think and you never actually work, then you might be "creative" but you aren't "creating." Alternatively if you just work and don't step back to think, you're creating but probably not doing it creatively. So you have to get into a cycle of create, observe think; create, observe, think.