A creative idea book is useful for every graphic artist / designer. It is a book that is used to record the ideas for the all the various problems.
Even if these ideas were not the solution for that problem and were not employed, you can have the idea book at your fingertips and flip through it time and again. This way, then-discarded ideas can give you the thought-provoking impulse for the solution to a current problem - or even be the solution itself.
Many designers have used such idea books since their study years, and still benefit from them later in their careers.
This idea book can contain everything that occurs to you concerning a problem that needs solving - from drawings to jokes, sayings and slogans to images.