A couple of years ago, there was a great post on Design Observer about Michael Bierut’s notebooks:

The post made its way around the blogosphere so extensively that I never bothered to link to it here. But more recently, a reader asked me if I had seen it, and it got me thinking again about working these notebooks into a post. Now, I finally have a good excuse as there’s a video posted of Michael Bierut talking about how he uses his notebooks:
[Updated May 2018: the original post seems to have been removed, but I think the video is now at this link.]

Check it out, it’s an interesting look at a designer’s creative process.