Dieter Rams 10 rules of good design.
designmuseum.org
These principles are timeless and are as applicable to idea generation as they are to design. Dieter Rams designs were simple, beautiful and functional too. One of my biggest takeaways from this was to make sure you develop, over time, a strong set of principles for your own work which you can return to when you need reminding of what good work looks like to you.