Okay, that clears it up for me. I was wondering why you weren't having the what-will-the-book-look-like convo with the publisher. (Uh, not that publishers really allow authors to have all that much [or any] input on the design.)
Do you anticipate different audiences for the less-graphic-intensive book (the one produced by the publisher; henceforth referred to as "the book") and the more-graphic-heavy supplement (self-published by you; henceforth referred to as "the supplement")?
If you anticipate (or hope for) overlap in audiences, I feel very strongly that the supplement should have some graphical elements that are the same as/similar to the graphical elements in the book.* Those tie the two together in the readers' minds, even if they don't consciously think about it.
(This is my own opinion about design; I believe it's the prevailing opinion, and one that's taught in design classes -- that you should tie produced pieces together visually. Obviously not all designers believe that, nor do they do it. And of course, the people paying the designers don't always want it, for whatever reason they may have. All that said, it's something I've always suggested with clients for whom I've produced more than one piece. Sometimes it's been as simple as keeping a specific Pantone color and using the same distinctive font. It's a principle Chatty and I try to push at work, because we want readers of our journal to associate our textbooks with our journals, but it's a losing battle with TPTB. They love everything to look radically different from everything else, and, in the end, they're the ones who sign our paychecks. I simply refuse to put the textbook with the blue cover and teal text [I am not lying] in my portfolio.)
*And having similar graphical elements might not be possible for you, if the book isn't completed (or even in production) before you begin production on the supplement. If you don't know what the book will look like, you can't retain some graphical elements for the supplement, and I get that.
Something to think about.