A notebook can play many roles. It can be a home for important information, a tool for keeping track of responsibilities or a nonjudgmental space in which to express the most shameful of feelings. This last use is arguably the notebook’s most revolutionary function: It’s easy to forget that we haven’t always had the ability to pick up a pen and jot down observations personally important to us.

In fact, it has taken centuries for the notebook to become the distinctly individual possession that it is today. In his book Writing the Self, historian Peter Heehs draws a parallel between the evolution of the concept of the self and the rise of self-expression through first-person writing. Heehs reminds us that “over the last two millennia, the prevailing idea of the self has changed f...