There is exposition, to be sure, but most textbooks are written so they develop the theory first and then they do brief insets that are meant to illustrate the theory. The insets are there for that illustration purpose as the primary and often they don't otherwise engage the students.
So I've opted for an alternative where I try to write essays that are related to the theory in some explicit way but the tie to the theory is looser and I hope the essays are interesting enough on their own. They are longer than those insets and because they are done online there are hyperlinks to other content so the student is encouraged to go off on any tangent that piques her curiosity.
The theory is also done differently. But that will be explained in a subsequent post.