Hey Hadley. Huge fan of your work! Many of the libraries you have authored or co-authored have had a big influence on how I think about building tools. I looks forward to getting a hard copy of the book!

I have a bit of a nitpick about chapter 13 on "relational data", in which I believe you are consistently misusing the technical term "relation" to refer to the relationship between two data sets. In the context of relational database theory, "relation" is just another word for "table" (although it connotes more mathematical formalism).

I think it is worth respecting the precise technical usage in this case: Consider a student who might read your book and be told that "relations are always defined between a pair of tables," and that "a primary key and the corresponding foreign key in another table form a relation." The same student might also stumble across the wikipedia page for the relational model and learn that "a relation is defined as a set of tuples that have the same attributes," and that the relational model "organizes data into one or more tables (or relations)."