I actually work the other way around. I start with something I want to say—an idea or story or image I want to share—then I try to figure out the best form it should take. Thoughts about a friendship might work best as folded notes in pockets. A story that takes place inside of a building might be enhanced by an architecural, shaped accordion or it might contain pop-up rooms. The life cycle of a plant could be shown in a revolving square or cross flexagon that has no beginning or end. Each of these ideas focuses on a single event, place, or object, then gets linked to a structure.
Granted, I've made many blank books that I've wanted to fill, too. Occasionally I make a complete book by tailoring what I want to say to fit the binding and materials and colors that already exist. I study one of these books, pay close attention to it, and see what it suggests to me, like my own Rorschach test. So I can start from the inside out (what I want to say) or from the outside in (what I want to make).
Hey, it's National Novel Writing Month! For content, try writing a one-paragraph novel. Include a place, several interesting characters, a character's want, need or desire, a change over time, either physically or psychologically. Then give it a physical home in a book form.
I have had a copy of this content-related comic from xkcd.com hanging on my bulletin board for months. When I read it for the first time, instead of the word "blog" I read "book."